Sounds like the unloader valve is gummed up with calcium deposits. That and iron are always issues especially with hard water. The best course is to replace the valve, it will be pre set from the factory. You might be able to disassemble it and soak the parts in CLR or something similar. You will need a pressure gauge to properly re adjust it.

cg, If this is only a zone wide problem, then take the valve back to the seller and replace it with a new one. If it is a system wide problem, then flow rate and pressure of your system are essential facts in determining problems. Aloha

I have a sprinkler valve on my system that will only turn on with the timer if I turn the water supply to that valve off and then back on once the valve opens. My husband has already replaced the solenoid and opened the valve bonnet to make sure everything is clean and not clogged or damaged. The valve works fine by manually turning it on with the screw. It also will work with the timer if I turn the manual screw on then off while the timer is on. I do have pretty high water pressure and am wondering if high water pressure could cause the valve not to open with the timer or if maybe there is some other reason involving the internal parts that I might not be seeing. Anybody have any ideas? Thanks!

I just wanted to adjust mine again to make sure I told you the right thing. So I took pictures and put them in paint. Once you take off the handle you will see a little screw. Unscrew that and you will get to the 2 plastic pieces. Those have grooves in them. Moving them closer together on the left will make your water hotter. Your handle will turn further to the left once it's adjusted. I only moved the pieces a few grooves and that makes a big difference. I attached a picture of mine to show you. Mine is a Moen. If you have a different kind you can look up the manual to see how to adjust the temperature. You could take off the handle and figure it out. Like I said earlier, the plumber told me how to adjust it once my inspector left. Hope this solves your problem.