A small gap is necessary because the brush head must be free to move in order to deliver the brushing action needed to clean the teeth. However, the brush head should not be loose and slip off your toothbrush easily. If this is the case, nor should it slip down the drive shaft during the course of usage. If this occurs, it can often be corrected simply by installing a new brush head on the handle. If the looseness persists after trying several brush heads, or the drive shaft is loose, contact Sonicare support to discuss your issue.