Hi gandalf42,
As randallandchris said, make certain the drain hole and gasket seam is down.
Maybe this was just covered. A risk of repetition.
MOST CRITICAL......
"Before" putting the cover back on, unscrew the handle a lot so the back-side lever is far from the cover. Just before placing the cover against the wall, that lever must be to the "LEFT". Then start screwing/tightening the handle to the right. That lever will immediately make a 1/4-turn and stop by hitting the end of the notch in the pipe, seating itself properly. Don't allow the cover to move....Keep turning the handle to the right until it is won't turn anymore. Then set the handle and lock with the key. Remember that if the key won't turn, loosen the handle 1/2 turn and the key will work. If the cover is not properly aligned, unscrew the handle just enough to allow rotation of the cover.