We have a 15x48 pool. This is the 2nd year 3 to 4 legs come off the ground. The legs don't seem to be moving on the other side. We've had our same liner for 4 years. Could it be stretching? Poured a concrete slab for the pool. First 3 years it never did this. Now this is the second year of "liner rolling". Any suggestions on cause and prevention?