Mine also has an end cap that’s loose. If I grip it around the cap instead of in the middle, my fingers alone creates a decent seal. A complete seal makes for a really intense and pleasurable suction. For that a big piece of plastic wrap wrapped around the closed cap and the top half off the Fleshlight tightly does the trick. Then you can easily remove it when you want to go back to having the cap loosely open with no suction, so the Fleshlight glides with no resistance, which is amazing after a few minutes of really intense suction. Also, I’ve used a condom that had expired that I was just going to throw away over the end cap instead, that works really well.
What I do is get myself all the way in so little air is left in the chamber, put the wrap on the case, and I hold it like I normally would, but my hand keeps it from slipping off as I have at it. A tight rubber band works also to keep it in place. Just be careful when you thrust out because after I pull out about 3 inches, the remaining 5 or so inches gets vacuum sealed tightly in the chamber (which feels really good), but if I pull out any further, the insert will get sucked into the case and make a really loud fart noise (lol). When that happens I have to pull out all the way and readjust the insert before I can continue.
The ring on the other end on mine is a little lose, but that’s never been an issue. If yours won’t stay put, I’m not sure it would function properly. Get some Flex Tape, that shit will fix anything.