My long haired cat has terrible knots help!?

I have two long haired cats that I do groom. However, one has very thick dense hair and the kitty brush just doesn't get right in, underneath. My cat hates grooming for any length of time, and now yet again has belly knots. I will have to get him shaved again, but in the meantime, how can I prevent this? any suggestions on good cat brushes for extra long haired cats?