Question
My apologies for my lack of knowledge of the correct terms, but here's my problem.  Our irrigation system consists of a holding tank with a pump down at the ditch and a pressure tank and controller up at the barn.  I've been running 5-6  sprinklers at a time and having no problem, but it has gradually decreased and now I can't seem to keep the system going for even one sprinkler.  When we checked down at the tank, the pump was running all the time, but not building pressure at the tank at all.  We have water at the faucet at the pump down below, so we believe that part of the system is working fine.  What could be going wrong up at the top?

Answer
I think the problem is with the pressure tank.  What to do depends on the type of tank you have.  If you have a tank with an inflatable bladder in it, you need to make sure that the bladder has the proper amount of air pressure in it.  You will need to consult the manufacturer's recommendations for the proper pressure, but it is usually about 2 pounds less than the cut-out pressure on the pump.  Also the bladder could be torn, and in that case the tank would need to be replaced.
If you have a non-bladder type tank, it could be waterlogged, meaning that it is filled all the way with water, and there is no air cushion to maintain the pressure.  You would need to drain out about 1/2 to 2/3 of the water in the tank to allow it to have the air it needs.