It's also possible that the bladder is busted. Is that happens, all the air behind it escapes into the shock body. You'd know if this happened, you can hear and feel the air bubbles as you compress and release the shaft.
There's a washer underneath the piston. Sometimes, if you tighten the bolt on top too much or if the washer somehow otherwise bends or whatever, it tends to get stuck at the bottom most position. This happened on a couple storm pro shocks I have. I just removed them, flattened them out with some pliers, and put them back on.
If they only get stuck when fully extended, then I'd just make sure you mount them in a position where they don't fully extend. If you put them in a spot where the suspension arms are stopped from fully drooping by the shocks themselves, then you may be putting too much stress on them causing trouble. The arms should stop on the chassis or the stop pins you can tighten.