Make sure the edge of your bed isn't resting on something it ought not to be and/or doesn't have any crap stuck to the underside of it. My Qidi has two plastic tabs sticking up at the rear which are supposed to be end stops to assist you in lining up the magnetic plate back onto its base, but if you're not careful you can wind up with the back edge of the base siting on top of them which has the net effect of making the build surface the equivalent of about a 1:64 scale skateboard quarter pipe. This has predictable results if you try to print anything on the back third of it or so.