New liner slightly sagging in two corners

I had a new liner installed in my 12x24 IG pool this past Spring. All summer, in two of the corners, the liner keeps wanting to come out of the "catch" or whatever the thing is that holds it in place. I can somewhat easily get it back in just using my hands...but then within a few days, its back out again. When it is out, it doesn't droop that much. The one corner is where they had the vacuum for a couple days making sure there were no wrinkles, so I assume that might have stretched it out some. Do they make some sort of shim or something that I can shove in there to keep it in place? Should I complain to the installer? It is under a one year warranty for installation labor. Any help appreciated. If necessary, I can take a couple pics.

Yes. The shim is known in the industry as "Liner Lock" and is sold by length. About 4-6" is good for each corner and is made specifically for that. Your installer should have known about it and used it as a matter of course. I'd call him and have him come back and install the liner lock. At no extra cost to you.