Forgive me for saying so fella, but that cover does not look all that flat to me. Does it rock from side to side when you press down on the sides away from the hinge and clip?
PS. Given up fiddling with mine. Flow had gone low again today, when I checked the impeller cover was out of shape again. I think this is a design fault, the cover being secured at only two points. In all other filters that I have had in the past that cover is held VERY firmly in place. I brought this filter because of the amount of time I was spending servicing my two ancient Sera 900s. I'm spending more and more time faffing around with the 406. Nufs e nuf. I'm going for a Ehiem Pro 600.