I can almost guarantee you that it's your shampoo. I had the same problem when I was using drugstore sulfate-free shampoo. It didn't have enough conditioning/detangling agents in it and the hair was always a mess. Then when you go to comb it out the hair breaks and some of the knots come loose.

Another suggestion- only use the brush when the hair is dry. Use a W I D E tooth comb. If you do run into t tangle you can usually "roll" the comb forward to gently remove it. Save the brush for dry hair only.