The shortcut "ALT K" does not work when the system is in play

[deibisantafe]

Goodnight everyone.
I have relatively recently a Protools HD 10 (official of course) running on an HP XW8400 PC, with two PCI-X cards, one HD Core and one HD Accel, as well as two 192 i / o, one analog and one digital .
Everything works without problems except one issue: track monitoring.
I have been with a Digi 003 for years, and the ALT K shortcut has always worked to alternate between incoming signal and signal recorded on the track, whether it is stop or play. All right. Well, for some reason I don't know about the system I have now. ALT K doesn't work once the system is in play. The green light of the "I" icon turns on and off if I press ALT K, but the monitoring status does not change, the one that was selected is kept before pressing play.
Any ideas? I'm lost
A million thanks