Keyboard Assignment problem PMDG 737s

I have been trying to assign a keyboard shortcut to 737s. I want 'increase autobrakes' to work with Ctrl+Shift+M. I deleted the default assignment, as I never need that as a shortcut. At the same time I assigned Ctrl+Shift+N to 'decrease autobrake'.The latter is fine, but oddly every time I assign the former, it works once, then it disappears from the PMDG keyboard options and shows up as blank when I open that window again. Every time I reset it, the software erases it again.Anyone know how to make this assigment 'stick'? Beats me why Ctrl+Shift+N is OK, but +M is not...Thanks,Martin S.