Might be a connection problem. I'd look at the positive pin on the MVP. They're known to have a soft insulator that can get crushed down and not make a good connection.
Power the MVP off, then using a toothpick, small screwdriver, or whatever fits, rock the pin gently from the outside in, to lift it just a hair.
Good luck!
The above is the firstest, quickest, easiest... and often a most temporary fix.
innokin had (still has?) a bad habit of building APVs with the 510 pin set too deep into the negative flange. On brand new MVP2s, I've measured them at .010" to .018" deeper than what might be considered "average". With the worst of them, it doesn't take long for a poor contact condition to develop.
The permanent fix - if prying the 510 pin up doesn't last long, or work at all - is to file, grind or otherwise machine down the 510 negative flange... which in effect raises the 510 positive pin.
Depending on the atomizer, you may have to remove a similar or greater amount from the beauty ring as well. If using bottom breathing, eGo attys, you may have to file new air grooves into the negative flange.
As it's all chrome plated brass, although you'll find a file will clog up quickly without a file card (file cleaning "brush") to keep it clean... a fine stone table top grinder does the job quickly and cleanly with only a bit of file and/or sanding block touch-up. Of course, a vertical mill does fine work... but most don't have mills handy. ;-)
While I'm on the subject... if air flow is poor, you can improve it greatly by drilling
through, and enlarging, the beauty ring air ports (using it as a guide) and through the 510 negative flange. On my MVP2s, I ended up using a 5/64" (.078" or 1.98mm) drill bit.
