Okay, I don't even know how to explain this wihtout showing pictures.
Did you make sure the backpack hatch is DEPRESSED underneath the 2 small locking tabs ? If the hatch is just lying flat to the back, then you've got it wrong. There's 2 little tab like fins on the back. One is part of the hatch, and one is on that little swinging piece of plastic. Make sure the swinging one is all the way down and not poking through the hole in the backpatch hatch, then depress the hatch PAST the level of the rest of the back and lock it into place underneat the two tabs on the right and left sides.
Dude I totally missed that feature, I guess until it was kinda not needed but with the fast pack out I am happy to know bout it.