I am a carto kinduva guy, and in the last six months have found two brand new cartos, that simply would not screw in. Once was after I filled a new one with juice, since then I do a test-screw to make sure it mates up properly, before I devote liquid to it. (there has to be a joke in there, somewhere...)
I know they are cheap (I use Boge 3ohm and some Smoketek dual coil), and the threads on the cartos are aluminum. Sometimes questionable QA, and aluminum threads? It's easy to understand some don't work, but hey, they are cheap.
Recently *every* carto has been loose, I checked them on something different, lo and behold, the threads are getting worn in my 510-510 adapter itself. It is made of harder stuff than the aluminum carto threads, I wouldn't have thought that it would wear so much, this quickly. Yet, I carry Darwin in the front pocket of jeans a lot, and DCC are longer, giving yet more leverage and strain on the connection.
My Darwin was before they all came with 510-510 connector. Before I ordered I knew the main connection was brass, and knowing something (even very little) about metal, I just wanted to have the daily wear and tear on an adapter, not the less replaceable brass threads. Turns out, that was a good idea!
I had ordered a couple of spare 510-510 from Drew long ago, just to have. That, and perhaps the packrat gene, I am not sure. I know the newer ones come with one, but it might be a good idea to order a couple from Drew, just to have around. They're cheap, and sure look better than duct tape, if it does get too loose to keep a connection.
This is the first time mine had been off, it was also a good time to get the q-tip and ISO after the brass connection, as well.