Just a thought, have you double-checked that the OD is, in fact, not engaged? I ask only because I had a friend who had a short in the control switch (so it was engaging all the time) and he complained that it didn't work.
Otherwise, I agree, time to pull the cover and check out the solenoid operation. I've had trouble in the past with the adjustment slipping over time (and repeated operation). So it might be out of adjustment now even though it was working fine.
Another possibility is that the pull-in contacts (or coil) aren't working. That can sometimes lead to a solenoid that will move when there is no pressure against it (engine not running) but won't make it once the OD has built up it's 400 psi or so (which the solenoid has to overcome).
BTW, it is possible for wear to make the factory adjustment method be inaccurate. The Buckeye Triumphs article at
https://www.buckeyetriumphs.org/technical/AOD/AOD4/AOD4.htm (about 2/3 down) gives an alternate method that is more complicated, but also more accurate (and compensates for any wear in the cam and plunger inside the unit). FWIW, I use 1/32" (.030" ) rather than the .016" the author mentions. I've checked several ODs with relatively low miles, and the factory adjustment results in more like .030" movement at the ball.
Also FWIW, I eventually got frustrated enough with the adjustment slipping that I modified the clamp a bit to make it more practical to adjust from under the car. A (hardened) socket head cap screw, plus a custom nut that engages with the clamp (so it doesn't have to be held with a wrench).