1. Check if the brush head is inserted in the correct direction. Try rotating the brush head 180 degrees and refer to the manual for proper installation.

2. The brush head needs a snug fit with the motor shaft, so it may require slight force to attach.

3. Inspect the inside of the brush head for any debris that might be blocking insertion.

4. Check the motor shaft for any obstructions that might prevent the brush head from fitting.

5. Ensure you are using only the brush heads designed for this product.

If the issue persists after these checks, please request after-sales support from the purchase platform.