Diagnosis
This is usually caused by hard water minerals or toothpaste residue acting like glue in the connection between the metal shaft and the brush head's plastic collar.
How to Fix It
Don't force it—you could snap the shaft. Let's dissolve the bond. 1) Mix a solution of equal parts white vinegar and warm water in a small cup. 2) Submerge *only* the connection area—the stuck brush head and the front inch of the handle—for 15-20 minutes. Keep the water away from the main body and button. 3) Remove it, dry the grip area thoroughly with a towel. 4) Now try to twist and pull the head off with a firm, steady motion. The vinegar should have eaten away the mineral deposit.
If That Didn't Work
If it's still stuck, the plastic might have fused. As a last-ditch effort, apply a few drops of rubbing alcohol around the seam where the head meets the handle. It can seep in and break down other residues. Wait a few minutes, then try again with a gentle rocking motion (twist left-right while pulling).
Heads Up
To prevent this, always remove and rinse the brush head after each use, and give the handle's metal shaft a quick wipe and shake to displace standing water. Let it air dry separately.