I take it you're using the prebuilt nickel coil heads from Kanger/vaporshark? I have had both and was never happy with the quality control on them. I'd look down into the atomizer head to see how the coil was wrapped and noticed on a lot of them that the
coils were touching and also never liked how they were held in by the blue rubber grommet insulator thing(the more solid of a connection between the
tank and device the better for temp control.) I noticed that with some heads I would never reach my my set temp limit(420° at 40 watts barely got me above 300° sometimes.) I stopped buying them and have been making my own in my lemo2 and Kanger sub mini with the rba. Plain Jane 28 gauge 9 wraps about .11ohms. Made sure to curl the wires under the screws and tightened them till they smiled. No problems since. Vapin at 470° 32 watts on my VS
dna 40 right now, what a treat!