Force Quit does not work.

Force Quit on my Mac does not operate. I press the keys "Command Option Escape." Nothing. No even a system-alert chime. I try the pull-down menu, Apple>Force Quit. Still nothing. The Mac does not obey my command.

The only way I can force-quit is to open Activity Monitor. There I can use View>Quit Process.

Activity Monitor is more cumbersome than the old familiar keyboard short cut. What does it mean when Mac OS no longer recognizes "Command Option Escape"?

(What I am trying to quit now is Excel. Something happened to my copy late last week. Now it is almost totally frozen. I can open one worksheet after I start the Mac, but afterwards I cannot do anything with Excel. I cannot even close the open worksheet.)