I love my iRobot s9. But this is the 2nd time I've had to replace the rubber brushes. What's up with the quality of brushes?? Please make the rubber more durable! The first time, a long string completely severed the brush into 2 pieces. Second time, they've worn down so the grip is gone. I love Roomba, but these brushes are too expensive to keep replacing. Pls fix!!