Is a “good” aluminium washer the same as a new one? There can be a difference. I always use new washers, and never have any trouble. Also, make sure there is no grit or grot under the head of the screw or in the hole on the cover – maybe clean it out with something like a wooden dowel, which will pick up the dirt and polish the seating a little. If all that fails it calls for much closer inspection, probably with the cover off.