I had the same problem. All I could figure was that the new texture of the hair coming in just didn't "mix" well with the old texture. ???
Clearly I'm no expert.
But trimming off some of the old ends helped me, and, I know I keep recommending this, but....COWBOY MAGIC. Those products made all the difference.
Good luck! OH, and, I find myself using a comb way more than a brush. That really seems to get to the "root" (no pun intended!) of the problem, whereas a brush just smoothed the "outside layer", and left those deceptive tangles underneath.