What causes dead spots in breadboards?
People regularly "borrow" my breadboards, then return them not working well.  What I've found is that they are plugging in wire leads or terminals that are too fat.  This causes the spring contact inside to warp out of shape and it no longer grips thin wire leads properly.

One of the worst things you can do is plug standard 0.025" square header pins into a breadboard.  Same with thick resistor or capacitor leads.  Doing so will stretch the contact pin out of shape and render that hole useless for use with thin component leads.

TO-220 devices will WRECK breadboard sockets UNLESS you to a really simple trick: grab each lead near the package just below where the lead gets wider and rotate the lead by a quarter turn.  Make sure the turn is gentle so as to not weaken the lead.  

If you look closely at the lead coming out of a TO-220 package, you will see that it is thinner than it is wide.  Unfortunately, putting the device into a breadboard such that each of leads is in a different column means that the thick portion of the lead is destroying the breadboard socket.  But if you rotate the lead, now the thin part of the lead is what spreads the contact apart.  FWIW - I'll usually also trim the ends of the TO-220 leads at an angle so they enter the breadboard more easily.

In terms of repairing breadboards, I've repaired a bunch of mine.  In days long ago, I could (and did) order replacement contact strips from the manufacturer.  Now that most breadboards come from the far East, I simply use a brand-new breadboard as a source of parts for fixing several wrecked breadboards.  The contact strips come out easily once you remove the self-adhesive paper label that covers the bottom of the breadboard.

Quick tip: use the lead of a 1n4148 diode to check each and every hole of any suspect breadboards.  If the lead isn't gripped tightly when it goes into the hole, that hole has been stretched and needs to be repaired.

Edit: I've been asked previously what one might use instead of 0.025" square header pins when plugging PCBs or carrier boards into a solderless breadboard.  I use a header strip that has 0.018" round pins instead of 0.025" square pins.  I've been using a Samtec part #TS-132-T-A (available from Digikey Samtec TS-132-T-A but someone on the PIClist pointed out that Amazon has what appears to the exact same item for significantly less money.  That link is here: Round Pin Headers on Amazon