Im no pro but maybe I can help....

If the liner is not new, it may have lost its elasticity. Usually, a liner will stretch in temps above 80. When I had to do a repair in my pool, I had to hook up a garden hose to my hot water heater and sprayed the sections I needed to get back on. The hot water will assist the liner in stretching. The other thing is they make "liner lock" or "bead lock". You insert this behind the bead to prevent the bead from slipping out.

Good luck