You need to make sure that the threads on the barrel are rotated so they are down - if they are up the smooth section will be down and the cap will fall out of the holder. The threads in the down position hold the cap to the holder. Here are some pics...I'm having issues getting the filler cap to slot in the holder and STAY THERE. I never had that issue on my other cars and its quite baffling how it doesn't seem to snugly fit into that "cup". Any tips?