Is it working or NOT ??:mad::ba:
It is for me but since I’ve no idea what operating system or version of blender (and build no.) you are using and what you are trying to undo, this may be of no relevance.
Maybe the first step is to learn how to ask a question ?
Win 7 x64, graphicall Build x64 http://www.graphicall.org/110by tungerz.
Can not recognize the rule on which Blender enables steps back or does not. Sometimes simply deleted mistakenly(or wrongly rotated) objects can not be undone.
PS. Sure asking a question happens to be a challenge but they say: There are no silly questions-only silly answers ! 
Make sure you are in the same mode edit or object as you want undone. Make sure it is checked in user preference editing.