I had a similar issue with my subtank. It was caused by two different issues.
The first was for some reason the pin on the bottom of the tank was not making good contact with the 510 pin of my mod. This made the resistance jump around and my APV pumped in way too much voltage for the resistance I was using and cause a fierce burnt taste. I replaced the 510 pin in the mod and this fixed this issue. I had to replace the coil too as once they are burnt, they are pretty much done.
Second was poor wicking. Even with those big holes in the head, they just weren't keeping up. I thinned my liquid with a few drops of vodka and it started wicking much better. I now use max VG with about 5% vodka or about 7-10% distilled water. I'm slowly weaning down my VG anyway due to other issues, but if your liquid is not wicking well for you, try thinning it out a little.
Finally, if you've already burnt any of the filler material, I wouldn't continue to use the coil head. I know they are expensive, but maybe consider rewicking it yourself or just replacing it with a new one. There is a very interesting read over in the General Vaping section about the coil heads on the Atlantis - check it out, then replace your coil head if you had any doubts before.