Diagnosis
Mineral deposits or debris have built up on the metal drive shaft inside the handle's neck, preventing the brush head from seating fully.
How to Fix It
Deep clean the handle's coupling. Remove the brush head. Use a dry toothbrush or a cotton swab to scrub the exposed silver metal post inside the handle's collar. Pay special attention to the base of the post. Wipe away all loosened debris.
If That Didn't Work
The retaining ring inside the handle's collar might be worn or damaged. Try a different, brand-new Sonicare brush head. If the new head clicks on snugly, the old head's plastic coupling was worn out.
Heads Up
Never use vinegar or other liquids to clean inside the handle neck. Moisture inside can damage the electronics. Only clean when the handle is dry and unplugged.