Take the cover off again and make sure the first screw is in the right hole. Don't tighten it all the way up until you get the second screw started. Figuring out lighting fixture mounting can be some of the most fiddly parts of electrical work.

If that screw hole happens to be stripped, just speculating, what would be the easiest way of fixing it?

What is it screwing into? The box or a bar across the center of the box? If the latter (which is a common mount) you can get those replaced. Most lamp repair or better electrical houses would have some (I always keep a few when I replace a fixture in my box of random junk parts in case I ever need one).