Un-mapped ATC from "T" and see if something else is mapped to "T". Just press "T" once you you have un-mapped, and if anything is assigned, clear that command out, and reassign ATC. When the last update (not this week), keybinds changed. Some had new ones assigned and may have caused duplicates. I don't know if "T" is default for ATC.
Also RCtrl + Enter while flying, and trying to engage ATC. Perhaps there is a spike from throttle preventing ATC activation.