My z failed to move completely, didn’t even hear the motor trying. Diagnosed my case as a Gshield failure by swapping the 4 Z wires and Y wires at the Gshield and attempting to jog Z up and down with the Y axis controls, which worked, while now the Z axis jog commands failed to move Y (be sure to turn off 24v supply power while rewiring everything just in case of stray contact here and there). My diagnosis isn’t entirely proven yet (got a new Gshield last night, plan to install this weekend) but you can do the same test.
If you’re on the stock spindle, might also test jogging with spindle not only off but disconnected to rule out a spindle going bad and mucking up with the digital signals.
Next idea, with a wrench on the pulley nut it can be hard to tell because you have good leverage, you say it manually rotates both ways but is it ‘harder’ to rotate one way than the other? Maybe you need a bit more power to the Z if you can hear the motor trying to move but failing to actually do so. Bump up the jog amount and see what happens and measure the direction that does work, did you get all the distance? If it’s missing steps it might just need a bit more power by twisting the Z potentiometer a tad. I kind of doubt that if it was working and then stopped…seems like unless you had wiring loosening up the ACME screw and delrin nut should only get easier to move with time, not harder…
My last idea is to watch the motor shaft carefully during jog attempts…maybe even make an offcenter dot on it with a Sharpie. Any chance it is turning but the pulley isn’t, in one direction? Might be a loose grub screw or whatever those little things are called…the tiny set screws that hold the pulley on the motor shaft. If it has a slightly canted edge it might ‘bite’ the shaft ok for one direction but slip in the other.