I suspect it the rubber diaphragm? It you rotate the solenoid counter clockwise the zone should turn on manually. This should let know if it’s the diaphragm (zone does not come on) or solenoid zone comes on. When I had zone failure with Hunter 100 globe valves it was rubber diaphragm plugged up/deteriorated and not working.
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The white plastic (in picture) if I remember broke/deteriorated not letting diaphragm to work. I have square valves with 4 screws holding body together. But, I live in older section and maybe using the larger 6 housing screws in newer areas?