Some caps that are made on contract [like the ones Dixie gun works sells are smaller than what the standerd nipple is made for;I have had to get the nipple padded with a bit of shim stock to protect the threads clampped in a drill moter and turned dowm by shaving material off with a file to get them to fit. Check to see if the cap is sitting down all the way on the nipple to see if this is the problem. Lee