First off, see if you have continuity to all the solenoids. Check with a VOM. They shold all be in the same area, resistance-wise. If so, make sure ALL the zones have voltage output in the 29VAC area.' You can check each valve when it's on with a small telecsoping pick-up magnet to check if it's actually fired. (You can feel the 60 cycle buzz when the magnet is lifted off the top of the solenoid when it's fired.)
Next step is water supply (are all valves/V.B.'s open. Last is diaphragms.
Being an unknowm system and the low cost of diaphragms, I'd replace them one at a time and see what happens....
Good Luck!!
The Irrigation Specialist Mfg' Az.
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