@myyetti , Isaac , First try another new original Oral B brush head to ensure it is not a faulty head. You may have to dismantle the tooth brush and clean around the small drive gear. Link #2, 3rd picture in step 4 of guide shows the small drive gear that may need cleaned, put back in position or replaced if worn. link #1 is a video showing disassembly of your Oral B. Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.

https://www.youtube.com/watch?v=PBYXM11d...

Braun Oral-B Professional Care 9900 Triumph Ni-MH battery Replacement