I have five sprinkler zones. Zones two through five are working well. Zone one pressure is so low that the heads don't even pop up. They trickle and flood the immediate area (a few square feet) but that's it.
The first suspicion was a broken pipe. These are normally very easy to find. The water floods to the surface and you can easily spot the problem area. There are no apparent leaks anywhere in the yard. I ran zone one for 20 mins straight and...nothing. I also searched for an unusually green patch. Nothing.
My second suspicion is a bad solenoid. I've never known a solenoid to "sort of" work though. They either work or they don't.
What are the other possible causes of weak sprinkler pressure?