I've had the exact same problem with both Kanger and Nautilus heads increasing in resistance. You have to increase the voltage on a VV device. VW PV's do that for you. Eventually, the coil will open up and you're done.
I think that so many people are vaping VW devices they don't realize that the voltage is being increased as the resistance goes up and up. My average life of a Kanger Mega coil, Kanger T3S, or Nautilus head has been about one day of vaping.
VW hides the issue by compensating for resistance changes. If every one would switch to VV and check the resistance when the vape falls off, we'd be able to document this problem much more widely. Both companies need to own up to the issues.
Now, for what I have found as a cause. If you pull the bottom pin once an atomizer shows open, you will find that one leg is broken away inside the atomizer. Both companies are using NR-R-NR construction (non-resistance-resistance-nonresistance coil construction) which is basically a coil with standard wire legs silver soldered to the coil on each side. These legs are soldered right at the left and right side of the coil where heat can break down the solder joint. Wire welding might be a better method.
I have enough bad heads that I can just micro coil them all over and over again until the problem is fixed in manufacturing.