At this point, the chance of your being able to remove and re-install the same liner are rapidly shrinking. It MAY still be possible but be prepared to replace the liner.

It sounds like your base isn't stable and there's water under there causing the sand to shift and wash away. I'm guessing on this.
You probably want to dig out the material under the pool, and fill it with crushed stone or QP (Quarry Product) and then fresh WET sand on that to get a smooth base. You may even be advised to put a French Drain under everything to move away water under the pool.

Just thinkin'