Never had a problem with them falling off…even with my 3 hour sleeps in a bath.
Swimming in pools (inside and outside) and in lochs hasn’t shifted it either.
Going on social media posts it appears they all fail but apart from the 3 I’ve hit on door sills (my own fault), I’ve never lost one and I’ve been using them since they first came out (self-funded the first 20 months before going on prescription).
I give my arm a clean with a swab and then let it dry. I then apply the sensor and as I press down, I then give it a hard press and hold down for about 5 seconds and then remove the applicator.
Seems to work for me, but aware it doesn’t work for everyone.