undo feature non-functioinal in Excel
 

Why is the undo feature not working in Excel? Both it and the forward
feature are "greyed out" on my toolbar. Undo only appears functional if I
have not left the cell. Once I hit "enter" and go back to that cell, Undo is
greyed out. I made sure my language was set to English, as suggested in the
Help section. It did not change anything. The file is not protected. Any
suggestions?