What I would suggest is that for the 7.4v double batteries, screw the adjuster out a bit so that when you screw the cap on, it screws them both down together. But I can get my bottom cap very tight even on the single battery, maybe try sanding the seating edge of the cap just a bit so when it seats it has some friction to hold it. On mine it feels like the groove cuts create a bit of friction.
4 and 5 ohm carts should be primed at low voltages and then hit with a semi-fast draw imo. You can't really treat them like 2.4 ohm Omicarts where you just hold the button down and draw super slow. To help you with hitting the cartridges we really need to know everything: type of concentrate, what ohm cart, draw speed, and button hold time.
Agreed. I have not burned out a single 4 or 5 ohm cart on 7.4v yet. I am drawing very fast and am modulating the button every 2 seconds or so to control heat.
to me it sounds like you are priming too much dry and maybe burning up the elements. Are you priming at 3.7v?