Small hex screwdrivers, or torx drivers, are often poorly made. You may find a more expensive set from a good brand name fit without any trouble--because they have been precision made. I have a set made by Wiha and they fit as if they were magnetic, which they are not.
You can also often free up a corroded stuck screw with a penetrant like PBlaster, not just a water displacement spray like WD-40. Auto parts stores carry these things. And also by soaking the parts in plain white vinegar overnight, or using a freeze spray on them. (Also in the auto stores.) If you use the freeze spray or anything else, just remember frozen plastic is easy to crack, let it warm up before banging things around.