I didn't vote in the poll because my iStick has two of the symptoms, but only one can be selected.
The upper threads of my iStick have become sloppy, despite consistent care in attaching and removing tanks. Also, though I've tried to refrain from over-tightening, tanks won't fire anymore when screwed on firmly. (The iStick reads them as either zero or Low Resistance.) The tanks aren't exactly "loose," but I have to stop as soon as the threads catch and the iStick display reads the correct resistance.
I've been using a 510-to-eGo adapter full time for awhile now and have a Kanger air flow control valve on the way, but there's no telling how long the female 510 connector on my iStick will last before more drastic measures become necessary.