Return jet not working in pool
Take a conventional garden hose and shove it in the faulty orifice.  If the fit is loose, try and shove something into the hole around the hose... and.... turn on the spigot!

One of more things will happen:  

1) You'll see water coming out of some other area.
2) The water will begin spraying back at you.
3) You'll realize that pool installers are idiots.

If the water begins pouring out somewhere else, it will provide a clue of how the plumbing is ran. If it sprays back at you immediately and almost no water pours back out of the pipe after you remove the hose it is saying there is a blockage very near you... or it's capped off behind the wall.  If it sprays back at you after 30 seconds, it means the pipe is blocked/capped farther in.  When you pull the hose out, does a lot of water drain back out?  

It is theoretically possible to have a line get clogged... but it would have to be from a build-up of D.E. material or some other mineral deposit, as nothing big enough in itself to clog a line would ever have been able to make it through the pump strainer.  I know when I turn on my 3 HP pump full throttle it'll blow little crusted pieces of D.E. out of the return lines once in a while... so I suppose it is theoretically possible that some of those chunks could build up on themselves.

I am a licensed general contractor and have a great deal of experience in plumbing, electrical, and concrete.  With that being said, the 30,000 gallon gunite pool that I inherited with the house I bought two years ago has cost me a great many brain cells.  It is readily apparent that pool contractors aren't terribly good at any of the three.  To make matters worse, pools don't have to have any permits here.... thus allowing the horrible mechanical craftsmanship to continue.  

Trying to logically analyze an idiot tends to crash my mental operating system.  "Does not compute.  Does not compute.  Does not compute".