I just got a couple Efest and have the same problem. GRRRRR!!!!!
Don't know about 18350's, but I recently run into a couple of Efest 18650 button-top, which won't make contact in my provari - even though they work fine in the vamo.
Under close scrutiny they turned out to have a shorter nipple compared to other Efest IMR button-tops I have, no vent holes in it, and a spelling error on the label ("Rchargeable", without the first E)... Either they're Efest knockoffs (yeah, that's a nice joke), or Efest's QA dept leaves a little to be desired.
Either way, I fully understand you can find a lot of variation among (supposedly) identical (supposedly) Efest batteries
EDIT: here you can see the difference I'm speaking about...
View attachment 165110