Diagnosis
The metal shaft that drives the brush head has likely collected a hard ring of old toothpaste and mineral deposits. This prevents the head from snapping on all the way.
How to Fix It
Unplug the base for safety. Take a paper towel, soak a corner in white vinegar, and wrap it around the metal stem where the brush head attaches. Let it sit for 10-15 minutes to dissolve the gunk. Then, use a dry toothpick or a stiff-bristled brush (an old manual one works) to gently scrape away the softened residue. Wipe it clean and dry before snapping on a new head.
If That Didn't Work
Inspect the rubber O-ring seal at the base of the metal shaft. If it's torn, stretched, or missing, it won't hold the head securely. You’ll likely need to contact customer support, as that seal isn't a standard user-replaceable part.
Heads Up
After every use, rinse the handle and remove the brush head, giving both a quick shake to get most of the water out. This simple habit prevents 90% of this gunk buildup.