Dusty;
If you have the frosted glass lenses, some of them are a real bear to get off. The light itself is held in the ceiling with three spring clips. Some of them are also a real pain to get to come out. I have the best luck on the tight ones in removing the light from the ceiling and use a couple of the rubber jar openers. It helps to grip the lens and it protects my hands from the spring clips. I had one that was so tight that I broke the tabs off of the lens getting it out, but it still stays in place.
Good Luck,
John