I use a small flat screwdriver for the really persistent ones and try to pry them on the ends, sometimes they come off like this, but you can easily loose them, so it's nice to have some extras. Using two screwdrivers like this can also help, one to hold and one to push. There also two small notches on the inside where you can fit in the screwdriver perpendicular, and flip them off this way (easiest this way I think). You get better at it in time