I'm having this problem on a 2015 model Dyson Animal upright vac which does not have a prefilter. The bleed off port will open after only a few uses, even though the dustbin has been emptied each time and never near full capacity. The problem is a layer of felted dust clogging the screen on the internal part of the canister. If I pop it open and brush away the dust layer with the vacuum's brush wand, it immediately works again, but only for about 2 uses. A Dyson support girl told me that dust clinging to that screen was due to static build-up inside the canister and that rubbing the internal components of the canister with a anti-static dryer sheet would solve the problem. But dust continues to collect on that screen. So I'm about to send it back for warranty repairs. :(