Had to replace the brushes on the vac head mid season last year. When I went to vacuum for the first time this season, I noticed the head was sticking on the bottom again. Sure enough, one entire side of bristles are worn off (they weren't when I started vacuuming, but many were breaking/falling off). What is causing the bristles to go bad so quickly?? Bleach? Sunlight? Any brands better than others?