Main drain covers are held in place with two stainless steel screws. Sometimes the construction guys use their cordless drills to install the drain cover and if they don't have the drill set right, they strip the threads in the body of the main drain cover. Sometimes covers just come loose.
It's important to use stainless steel screws to hold the cover down. If one screw is missing, take the other one and go to Home Depot and get another. Try putting the existing screw into the other hole while you're down there. If the hole is stripped out, then buy the next size larger screw when you go to Home Depot.
It's hard to stay on the bottom of the pool to work on a drain cover. Pool guys use weight belts of the type that scuba divers wear. You'll need something to help you stay on the bottom. When you start exerting pressure on the screw to thread it in the hole, the action forces you up away from the drain. A good dive mask is also needed.
Don't tighten the first screw that you're installing all the way. You need some play in the drain cover to align the other screw with its hole.
If only one screw hole's threads are stripped, you can tighten the other screw and it will hold the drain cover in place.
If both screw holes are so bad that you can't use screws in them, you still have one option. You can make a ball of A&B epoxy and use it to anchor two sides of the drain cover. This isn't pretty, but it works. Turn the pump off for 24 hours to allow the epoxy to harden. If access to the main drain assembly is needed, the A&B can be chipped away with a screwdriver.
