Diagnosis
You're likely dealing with a battery that's lost its capacity and can't deliver full power anymore, or there's significant mineral buildup on the drive shaft.
How to Fix It
Start with a physical clean. 1) Remove the brush head. 2) Look at the small metal post (the drive shaft) that sticks out. Grab a cotton swab dipped in white vinegar and gently scrub that post and the area around its base to dissolve any hard water deposits. Dry it thoroughly. 3) Put on a brand new brush head (old bristles wear down and can *feel* less effective). Give it a test run.
If That Didn't Work
If it's still weak, it's almost certainly the battery. These lithium-ion cells have a lifespan. Try a full discharge/recharge cycle: run the brush on its highest mode until it completely dies, then give it a full, uninterrupted 12-hour charge. If performance is still poor, the battery is fading for good.
Heads Up
Avoid leaving it on the charger 24/7. Once it's fully charged (light goes solid), take it off. Constant trickle charging can stress the small battery and shorten its life.