The simple answer is that the HW2 coil is not a Nickel Ni200 coil, the mod is detecting this and defaulting back to standard power (VW) mode. To run in any TC mode you need the coils to made of very specific wire types.
Nickel Ni200 Mode = Need Nickel Ni200 Wire. Ni200 is very, very, very low resistance wire, is very soft, and has a very low melting point, but due to how low its resistance is, mods can detect resistance increase of the wire very very accurately and determine the temperature based on that resistance increase, that is how TC mode works (this wire can only be run in TC mode, get close to melting point, the Ni200 wire begins to enter the vapor stream and can lead to nickel poisoning)
Titanium Ti1 Mode = Need Titanium Ti1 Wire coils. Titanium is another low resistance wire that can be used only in TC mode like Ni200, it is harder than Ni200 and many other metals, get it to hot it will produce titanium di-oxide (think white pigment for white paint they use titanium di-oxide for that). Ti1 has higher resistance than Ni200 as well but lower than the other common wires we use, again it is very accurate to read the resistance increase as the wire is heated to calculate the temperature of the wire.
Stainless Steel 316L (SS316L) Mode = Needs SS316L coils. SS316L is the new kid on the block of commonality and very unique as it can be run in standard VW mode or TC mode, only wire were use that can be done this way, and sits in the middle of all 5 wires we use for resistance as well, just a bit higher in resistance per inch over Ti1, another harder alloy so hard to work with unless annealed like Ti1.
Most coil heads made pre-fabbed are made from either Kanthal A1 alloy or Nickel-Chromium Alloy, very high resistance per inch wire that can not be calculated accurately for TC.
