I thought it might be the batteries wearing down after use so I just bought 8 new batteries. but that doesn't seem to fix the problems.
the new batteries, after their initial charge was complete, did not give you a reasonable runtime and the vape you are accustomed to getting?
i'm not getting much vapor as when first new and batteries only last 1-2 hours. I tried using sand paper on the contact screw but nothing I do seems to help. any suggestions?
i keep thinking that your batteries are being over-discharged, and to some extent they can bounce back to vapeable levels (voltage), but they can also be ruined altogether and unable to sustain a charge. how long do your batteries last on the charger before the light turns green? do you have multimeter, and are you able to test the voltages of the batteries at any point, before during or after the charging process? are all of the newer pairs acting the same way, and if so was this after their initial charge too, or after a series of charges did the performance/ runtime decrease?
you'll need to check the voltages of the batteries. if you don't have one already, get a very cheap multimeter to do so, less than the cost of a new pair of batteries, and that will tell you a lot about what might be going on. unless you have the very first run of the V3 with the non-plated contact screw, you shouldn't need to do any sanding whatsoever. breaking the unit down and cleaning the threads, polishing the contact screw, and careful reassembly/realignment is really all that's needed. and you can dab a little noalox on your contacts or threads if it makes you feel better. but unless you managed to vape around 4 or more mL of juice in 1-2 hours with those batteries, it's likely that something is awry and the ability of one or more of them to hold a charge has been compromised.
let us know what's going on with your battery voltages, or at least
when their performance dwindled after you received them (i.e DOA, after first charge, after charging a few times, etc.), and we may be able to better troubleshoot from there.