Question
one head not getting water. removed head ,water sitting in pipe but none coming out when .  rest of zone apperars to have enough water pressure.  all other zones fine.  this happend upon start up this year.  no water puddling out anywhere, indicating obviuos crack.  maybe not blown out properly at end of season? i thought maybe could be ice but our high temps are in the 60 s now.  it is the last head in the zone. if debris is in the line could i try to blow it out?  or do i need to grab a shovel and start digging, either to find problem in line, or to replace section in line?

Answer
Dear  Rick, I hope this helps. Sounds like you're on the right track. First thing I would try, is stick a hose in the top of the threaded fitting down below, and try to force some water back against the flow of the water. Then turn the system on again and see if the obstruction passes through. If that doesn't help, then I would see it you can push a piece of wire, or an electricians fishtape, into the pipe through the top of the fitting to see if you hit an obstruction. Alot of time by doing this, you can pull the wire out, and determine how far over to dig, and not have to dig up the whole line. Let me know if I can  help you out anymore. Thanks and have a great day.