Why does mt Triple Bristle keep dying over and over again?

My Triple Bristle automatic toothbrush lasts 4 months then it dies completely. It has been replaced 3 times by Triple Bristle and they said it is the last time they will replace it free of charge. They say this has never happened before with their tooth brushes and they have sold thousands. My dentist sees a marked improvement with my gums since using this toothbrush. I have left it on the charger for the first two times it died, and, now this time I only left it on the charger for 24 hours then off until it needs charging again, Both the base and handset have been replaced, it makes no difference. Why is this happening over and over again but not with anybody else? The reviews are great!!!