Anyway you are most likely having a problem because you have the drywall coming right out to the corner, you need to trim those back about a 1/2 inch on each side.

It's either that or the corner isn't a 90, (check with your square) or you aren't cutting at a 45, or the planes aren't level. To make sure you have the 45 you can always put the bullnose over a 2x4 and cut through with a mitre saw... with it in position so it doesn't flatten out on you. (I'm assuming vinyl, don't do this if you are using metal)

I'm betting you have the proper set back at the corner.