My experiences with buildtak are very poor. But I have heard that the trick is to get the (first-layer) gap between the nozzle and the buildtak just right for a particular filament. Too small and you can't get the part off without destroying the buildtak, too large and it won't hold at all. As with Goldilocks it has to be 'just right'. But again, that is what I heard. After ruining two sheets with parts that would not come off without tearing the stuff, I went to glass and hairspray and never looked back. You could try to reduce the gap by 0.05-0.10 mm and see if that helps?