In my very limited time working with this software I've found that I can only "undo" something once.
If I happen to go two or three more steps beyond the certain change I've decided to "undo," it just can't be done. (at least not by me)
In a case like this, I'm still trying to figure out how to just delete or remove that particular section of my sketch/drawing that I would have "undone" if I could have.
Then I could re-do just that piece of the drawing without having to start over from scratch.
I'm sure you can you tell from my reply, just how new of a newbie I am.