Hair, string, yarn or any other fiber that gets wrapped around your brushroll is going to make it less effective. Remove these materials often. Use a seam ripper or a pair of scissors. Also, remove the end caps and clean the ball bearings on each end. After 4 or 5 years, you may find that the bristles are softer and shorter. When that happens, it is time to replace the brushroll.
Both the old metal floor brush and the plastic floor brush on the EXL, MG1 and MG2 TriStar models have replaceable bristles. They are very easy to replace. Don’t throw the brush away, just replace the bristles.