Hi, Welcome to JustAnswer. I am a licensed veterinarian and I'm here to help you. Are you by any chance referring to their dew laps? Female rabbits will have this fold of furred skin hanging from below their chins. It is their normal conformation, and it is from this dewlap that they pluck out fur to line their nests for babies.
Or do Velvet and Flopsy have mats of fur that clumped when shedding and now need to be removed?
I'll be waiting for your reply and we can go from there. Also, if you'd like to send a picture, that would be great!