Hub dynamo, maybe loose contact; maintenance?
Here's a guide for overhauling a cup-and-cone low-end Shimano dynamo:
https://dl.dropboxusercontent.com/u/34057344/Overhauling_Shimano_Dynamo_Hubs.pdf

The main caution from that article concerns the connector-side cone &amp; nut and the aluminum wire underneath them:


  Using your finger to prevent the plug assembly from turning, break the lock nut from the cone and remove the locknut. The plug assembly will try to turn with the locknut, but if you allow it to rotate while the axle is stationary, the wire which runs from the internal assembly to the plug will break and your hub will be ruined.




For completeness, here's the full article.  Duplicated here because the initial website is already gone, and this enables better indexing by the SE search function



Overhauling Shimano Dynamo Hubs
Shimano makes a nice, affordable dynohub used by many commuters. It isn't as light, efficient, or pretty as the Schmidt, but it is less than half the price and has been reliable for me as well as many customers. Unfortunately, Shimano doesn't provide  any  instructions  for  overhauling  their  dynohub,  and  the  technicians  at Shimano  North  America who I have spoken with are unfamiliar with the hub, offering me  incorrect advice about its disassembly. The Shimano dynohub uses traditional cup and cone bearings, but there is one hitch--there is a wire running from the internal assembly of the hub to the plug on the drive side of the hub. Care must be taken to remove the wire from the plug assembly so that the hub can be taken apart and the bearings serviced.

Tools
The only special bicycle tool you need for overhaul ing the hub is a 15mm cone wrench.  You'll also need a 17mm wrench, a 32mm wrench (a headset  wrench works well) or large crescent wrench that can open to 32mm, and a sharp tool used to pry up a couple of thin pieces connected to the plug.  If any of your bearings need to be replaced, the Shimano dynohub uses 3/16" ball bearings on each side of the hub.

Hub Parts
A pdf of Shimano's exploded diagram for the hub can be found here. (Broken link - if you find one please edit)  This diagram is helpful for visualizing the hub, but it won't help you to access all the bearings, because unless the cone from the drive side is removed--which requires the  removal  of  the  plug  assembly  and  detachment  of the  wire  from  the  plug assembly--the drive side bearings cannot be accessed.

Do I really need to do this?
The  dynohub  produces  electricity  by  the  rotation  of magnets inside  the  hub shell,  these  magnets  give  the  hub  a  "notchy"  feel  w
hen  spun  in  hand.  At  high speeds, this notchiness can also be felt on the bike. This is perfectly normal. The DH-3N71's bearings are very well protected by two seals on each side of the hub. I have used one of these hubs for several thousand miles before opening up to find the  grease  was  perfectly  clean.  Of  course,  your  results  may  vary.  Try  adjusting your hub using the non drive side cone if you feel the bearings are too tight. If you feel  the  grease  needs  to  be  replaced,  or just like to  take  things  apart,  use  the instructions below.

These instructions assume you know how to overhaul a traditional cup and cone hub.  If  you  need  instructions  on  overhauling  a  cup and  cone  hub refer  to  these instructions from the Park tool website - http://www.parktool.com/blog/repair-help/hub-overhaul-and-adjustment

Directions
Place your 15mm cone wrench on the drive side cone in between the plug and
the  hub  body.  Place  your  17mm  wrench  on  the locknut. Using your finger to prevent the plug assembly from turning, break the lock nut from the cone and remove the locknut. The plug assembly will try to turn with the locknut, but if you allow it to rotate while the axle is stationary, the wire which runs from the internal assembly to the plug will break and your hub will be ruined.



Now  that  the  lock  nut  is  removed,  you  can  begin  to take  apart  the  plug assembly  and  remove  the  current  wire  from  the  plug.

Below  is  a  picture  of  the parts on the drive side of the hub. Parts 3 though 8 constitute the plug assembly along with the current wire and the connector that is soldered to the current wire.




Rubber Seal
Cone with Dust Cover and Seal Ring
Back Plate of Plug Assembly
Metal Spacer with Wire Slot
Metal Spacer with Wire Slot (identical to part 4)
Plug Assembly Body
Plastic Plug Assembly Cover
Ground Plate
Lock Nut


Begin to take apart the plug assembly. First, use a sharp tool to lift off the ground plate.



Do the same with the plastic cover to the plug assembly.



Now the current connector and current wire are exposed. Sorry for some of the blurry pictures below.



Use your sharp tool to lift up the current connector and wire. Be careful not to stress the soldered connection between the two.



You'll notice an opening on the bottom of the plug  assembly body. This opening is large enough for the current connector to pass though. Carefully work the plug assembly body off the hub.



If this is the first time your hub has been opened, there will be a mess of white,semi-solid substance (which I assume is Teflon) slathered around the wire where it passes through an opening in the silver metal spacer at the triangular end. You'll probably need to pick away some of the Teflon to remove the spacers. Below the spacers,  the wire runs in the  groove on the hub axle.  After you've removed the spacers (parts 4 and 5), the hub will look like this:



Place the wire along the groove in the hub so that the back plate (part 3) can be removed and the cone screwed off the axle without damaging the wire.



At this point the ball bearings on the drive side are exposed, but they're not easily accessible and you'll find it hard to clean the cups of the hub well unless we remove the axle.



Break the lock nut from the cone on the non drive side of the hub and remove both. Then using a 32mm wrench grab the large nut on the drive side of the hub pictured below and turn counterclockwise. This will take a little force, brace the wheel against your body for a good grip.



Now the inner assembly of the dynamo can can be removed.



With the axle out of the way, the bearings on each side of the hub can easily be removed  and  the  cups  cleaned  and  inspected.  Remove 
all the old grease and repack, replacing bearings as necessary.

When reassembling the hub, remember to prevent the plug assembly from turning as you  cinch  down  the  locknut.  Clearly, all owing the plug  assembly to rotate while holding the axle stationary can break the wire.