Diagnosis
This is almost always a worn-out or damaged O-ring seal inside the toothbrush handle's shaft. That little rubber ring is what creates the friction fit for the brush head.
How to Fix It
First, turn off the brush. You'll need to clean and inspect the shaft. 1) Pull off the brush head. 2) Use a dry cotton swab to gently clean inside the metal shaft. 3) Shine a light in there and look for a small, flat black rubber ring around the inner wall. If it's cracked, missing, or looks mashed, that's your culprit. You can contact Burst customer support—they often send out replacement O-rings or even a new handle for this known issue.
If That Didn't Work
If the O-ring looks fine, the issue might be a specific batch of brush heads with slightly off-sized connectors. Try attaching a different, new brush head. If it snaps on snugly, the problem was the old head, not the handle.
Heads Up
Don't try to lubricate the O-ring or shaft with anything. Water is fine, but oils or soaps can degrade the rubber faster and won't solve a physical wear problem.