oh, you meant the lugs.
try with new screws. The screws are plated brass and quite soft, so the thread on the screws often goes before the thread on the steel lugs.
If you ever have a screw with a partially stripped thread, throw it far, far away to prevent re-use and future rage.
The brass screws supplied with the face plates have done this to me 3 times, but normal steel ones thread perfectly well in all of them, as you describe.
If I purchased brass electrical screws and had another go do you think they will work, or not?
Some of the face plates now have one silver and one brass, which looks duff if you ask me.
I'll have a go with a bit of nail polish.