Just wanted to add my experience last year to this thread. Yes, leaves and other "stuff" in the pool can definitely cause the hydrostatic valve to not close properly. I had the pool company open my pool last year. They did it on a Friday on a weekend when the family and I were out of town. We came back Sunday late afternoon and the pool was almost empty. Same problem, leaves got into the valve and it wouldn't close properly. I lost 12,000 gallons of water (22,000 gallon pool). Also my first year of owning an in-ground pool and had no idea this could happen. I have a gunite pool, so one of my option is to permanently close the valve. Since there is always water in the pool there shouldn't be a problem with the pool ever popping up out of the ground.

I haven't opened the pool this year yet, but I will monitor the water level very carefully once the pumps go on!!