Curious. There's nothing wrong with putting either Nautilus tank on any mod at any power up to whatever your juice will allow. For me, that's 14 or 15 watts for some, nearly 18 watts for others.
Your mod is (obviously) over-specified for a 8-18 watt tank, but that shouldn't be a problem.
Possibles..
Problem with the power regulation on the mod?
Vaping too fast? (unlikely...unless you have very thick liquid)
Strange (heavy flavor) e-liquid flavours that gunk quick?
Your iStick is reading the ohms at 2.0 or 2.1? (BVC 1.8) That's about normal for iSticks.
You ARE using BVC coils, yes?
It should all be working well, really.