Hi Tom,
Welcome to the forum.
If reassigning the hotkeys does not work for you then there could be an issue with your Mach3 installation. A re-install of Mach3 may cure the problem.
Before uninstalling the existing version and installing new...
Backup the following to the desktop, or even better, a USB drive:
XML(Profile) – This stores you’re settings for Mach3, if you lose it you will need to re-setup you’re ports and pins, motor tuning, etc.
Screensets
Macros/Brains
Tweakie.