I have absolutely no experience with floating liners (lucky) but simple physics says if you keep a positive pressure the liner should not float. I'd first try and fill the pool as high as possible, to almost overflowing, and this should make the pool water level at least a few inches higher than the surrounding water level. Even this slight pressure differential should slowly force water out from behind the liner. Water got behind the liner some way and it should be able to be pushed out the same way unless the shell is so tightly sealed this can't happen. I have a high ground water level especially in Spring with the snow melt and keep the pool level higher than normal and have never had a problem. I suppose you could have high pressure spots depending on underground flow paths and surrounding terrain and if this caused the problem just ignore what I said.
Al