Pistons won't retract when power is removed unless they're broken.

I'm using the pistons to hold back water in my farm so I can release it when I want to harvest the crops, but not all the pistons retract even though they're redstone wired exactly the same as the pistons that do retract. It also seems to change pistons randomly. Somethime all work, sometimes none work, most of the time only part of them work and which part changes too. I thought it was me at first, but its too random to be somthing I did.