Gunite pool leak

Gunite pool- painted last year- no cracks, in really nice shape.  Lost 1/2 its water- stopped a foot below the lights.  Dual <Maindrains- with hydrostatic valves (not too old).  My question is can a faulty Hydrostatic valve let only 1/2 the water out?

Cover collapsed from weight of snow this year- torn up bad.Its a Merlin cover and I have the original order info- can you guys order a Merlinn cover too?