my guess would be that they are not liking the brush. If you don't have a Zoom Groom, you might want to get one. It delivers a massage and a grooming at the same time. Have treats to offer when they accept the brush, so they can associate the brushing with positive things, and clip their claws prior to starting to brush them.
I would definatly try the zoom groom. Dori HATED to be brushed and I read a post where someone mentioned the zoom groom. I bought one and instead of brushing her right away with it, I sat it next to her a few times so she could become familiar with it. She began to rub herself all over it. Now I can brush her with no problem.