A:Answer i was (& am) unsure about brush installation.
in my case, i have about a 2mm space between the base of the brush head & the brush itself.
seems wrong, but it works fine like that.
if small pressure is causing issue, that is wrong, & i would exchange it for another, & when putting the brush head on, push it down & when it seems like it won't go further, stop (leaving the mentioned space).
(i had an older model Sonicare that uses a different style, screw-on type brush head, & with that one, when the head wears out, minimal pressure will stop the brush, & it is time for a replacement head. i purchased the Sonicare 1100 instead of a replacement brush head for my existing toothbrush - worked out cheaper that way.)