What version of photoshop and operating system?
So if for example you open photo and duplicate the background layer twice, then go to Edit, the Step Backward command is greyed out?
If it's not greyed out press the step backward and both step backward and step forward should be available.
Are the correct shortcuts listed on the edit menu. Alt+Ctrl+Z (step backward) and Shift+Ctrl+Z (step forward) and Ctrl+Z (undo)?
Do you happen to be using other language inputs? Double check your language bar.
Step Backward and Step Forward are unavailable while performing some operations such as cropping, transforming and entering text.