My suggestion is to create a keyboard combination for the Avionics Master Switch. I use CTRL+SHIFT+A and that works every time for me.
I have also found that if the engines are running I can press CTRL+E (default keyboard combination for engine start) as that automatically turns on the Avionics Master Switch. Off course you probably don't want to start the engines just to get the FMC to turn on.

Hope this helps.

Mark