Try a slightly thicker wick with a larger diameter coil, that will allow more liquid to flow. If you're still having issues it's probably at the Kayfuns limit of supplying juice through the channels.
I wouldn't worry about it though, you don't gain much if anything from going sub ohm on a kayfun and they aren't really made for it, the heat can mess up the insulators over time. I've modded mine where they sub ohm great but still I choose to keep my builds in the 1.2 ohm rage.