This has happened to me too. We replaced the liner in 04, the pool itself is 22 plus yrs old.

Rather than add water behind the liner or use a hairdryer or heat gun (they get REAL hot REAL fast and can melt it WAY too easily), the first thing I would do is to run hot tap water (NOT boiling) into a pitcher and pour it over the area. This should soften the liner enough to be able to pull it up into the bead. Then use liner loc clips to keep it there. This is all best done on a warm day.

I would imagine that the shrinkage discussed is more likely a factor if the liner has been out of the slot for a while. I am nervous about adding water behind my liner with my "senior" status steel walls, and in my opinion, the less water-where-it-shouldn't-be, the better. When I did it, it took me less than 10 minutes - and without my husband's help. I also have not needed the liner locs and it hasn't come out again in 2 years.