I would say the post aren't deep enough. A dogs ear makes an "L" shape, so don't be afraid to set the post in the bottom of the ear...
I think i am probably going to switch to this method....I will try to taper it. Gunner has small ear canals and no matter what I do, they keep popping out. Do you put cotton or anything in the ear to prevent pockets?Maybe use a backer rod/pipe insulation method. Taper down the bottom to make it easier for the posts to go in. Juno has teeny ear canals, so I taper.