Problem with valves

Just finished my install of 4 zones run off a shallow well. 3/4hp jet pump with a pump relay hooked up to my nelson controller. Has been working great for the past week. Now zones 3 and 4, the valves don't open (rain Pro 8410t jar top valves). I've checked at the controller and each zone gives off 24 volts (when the controller send the voltage). At the manifold (50' away) both zone 3 and 4 have power and i can hear the solenoids. I've taken apart the valves and nothing is restricting them. When the solenoid is activated if I manually open the valve a bit then close it the solenoid seems to hold open the valve. valves 1 and 2 work perfectly. i can't figure out what is going on. everything is brand new. thanks

Did you check the ground wire? This is usually linked from valve to valve and you may have lost the connection after #2 valve.

Thanks Gary, no the solenoid fires and retracts into the body of the solenoid when 24v comes from the controller. As I understand it when the solenoid engages it allows water to flow down the small stainless tube to the bottom chamber of the valve which then releases the diaphram and water flows. I can't for the life of me figure out why sometimes the valve opens and sometimes not. When the pump is sending water and the valve has not automatically opened I can break the pressure difference by manually opening the valve then closing it and the solenoid keeps it open. I'm stuck

Do you have a pressure gauge anywhere in the system?

Some valves have a minimum PSI requirement for opening. Sounds a bit like what you are experiencing.

Grasping at straws here without being there, but is your well possible putting out less water once it gets to zones 3 and 4?

Thanks, PSI from pump is 42psi. I'm pretty sure water flow/volume is not the problem. Is it possible for the voltage drop to not cause the plunger it retract with enough force inside the solenoid? I mean it sortof humms and if i disengage the solenoid from the valve body it retracts but I can't tell if it does so with the water pressure. What would be the minimum voltage to cause the plunger inside the solenoid to activate? 20 Volts? The thing is that these 2 valves work episodically and i can't trust them to come on at 5am, I would burn out my pump with no water flow for 20 min x 2 valves. thanks

Can either zone 3 or 4 be manually started from the controller, as the very first use of sprinklers? (well isn't drawn down)

Good idea, but tried that. No, it made no difference. I really think it is a problem with the valves or something inside them or some adjustment. they work just fine when i manually open the valve. i'm stumped.

I can hear the solenoid buzzing and if i give it a slight tap the valve comes on. These are brand new valves. Suggestions other than the obvious of change the solenoids? thanks

Are the valves threaded or glued into place. You can always start swapping the valve internals and/or solenoids. And you do have a spare valve for parts, don't you?