I just fixed the problem on my Roomba i7. See if the brushes roll freely when you turn the unit upside down (they should). If they don't, then you may have the problem that I had. In my case, brushes were appeared to be fine until I closed the side piece that holds the brushes in place. Removing the end caps (bearings), I found that the axle on one of the rollers was pushed too far into the round metal piece which then did not allow the end cap to rotate. Once I pulled the axle out and repositioned it, everything worked fine. This is also why getting replacement brushes will also fix the problem.