sorry if this is obvious but are you sure the machine was on? for me, sometimes i can't tell if my machine is on because the pressure settings are really well dialed in and I have to look at the machine's screen while using it to make sure(or turn it on in the app)

I am inbetween a medium and large for F40, I had a decent amount of blow out with medium, after switching to large the experience is a bit better, some blowout from the upper nose bridge, but i've found that if i move the mask down a smidge then the blowouts stop