Something strange

SOME of my Word documents do not recognize the Control(Ctrl) functions

You know, when you press Ctrl button, you get this :

Ctrl + A = Select All
Ctrl + S = Save
Ctrl + P = Print
Ctrl + Z = Undo

..etc.

These do not work when I am working on some files. But they work with some other word documents. I have no idea what the problem could be .

Any ideas ???

I know that mouse has made life easier, but being an old man, I feel more at home using the keyboard function than mouse menu.