Help! ACT is giving me a hard time - SOLVED

So I followed FBB's excellent video about setting up ACT in X1. Great - for the first time I understand how to do it, so thanks, Karl.
The problem is, I can't get the MIDI Learn to work in the controller properties pane. I click the ACT button - it's now lit up blue.
I click in each cell and touch the corresponding control - nothing happens. After a few seconds the MIDI Learn caption goes away but nothing is assigned.
I have selected the correct MIDI device (in my case AXIOM 61 MIDI In) in the Connected Controllers/Surfaces dialog.
I have selected the correct preset on the controller (memory location 20).
The controller is transmitting on MIDI channel 0, and any chosen soft synth is getting note on/off and CC data, so I'm sure MIDI connectivity to my PC is good.
I have verified that MIDI data is generated when I touch any control (using MIDI-OX), but ACT doesn't seem to be receiving it.

Any thoughts, good people? 

UPDATE: Rebooted PC - no more problem. Don't ask me why.