You don't say which Istick, or which coil for the Subtank. Low generally means the sensed resistance is too low for the device to power. The ST comes with (usually) a 0.5 and a 1.2. Manufacturing tolerances being what the are, the 0.5 might be a bit low for your mod. Try the higher value coil.
It's also possible that you''re just cranking the topper down too hard, crushing the insulator on the mod and causing the short. Snug is all that's needed.