Had a lengthy break from DCS but was still struggling with this in 2.5.6. Was determinted to fix it without resotring to trashing all my bindings, and won.
In a cold and dark jet. the default keyboard bindings would move both levers between OFF and IDLE with no issues. The warthog throttle would move them from OFF to IDLE, but it was impossible to go back to OFF again, even using the keyboard.
Seems to have been a simple case of the axis input not being zero when they're all the way back, and that prevents them going to OFF. I changed the X saturation for easch axis to 90%, and everything is good again.