The bottom of my 17-year-old inground gunite/white plaster pool has 2 round plastic drain covers, each with 2 screws holding them in. This winter for the first time ever, one of them came unscrewed - I assume from the pool vacuum, which often pauses on top of them when it's on. One screw was lost but I was able to fish the cover out with the other screw and I bought a new matching screw. Now I wonder - how will I put it back on? Wait until the water is warm in several weeks and do it myself? The other thing I'm wondering is, will the screws stay this time or have the threads been stripped? I have no idea what they're screwed into - or if there is something additional I need to do so they don't come out again. Has this happened to anyone else? Advice? Thanks!