I just did one and it worked fine. Have you tried hitting the tab to move down, make sure the change was done? Also, what happens if you save as draft to finish later, then try to do it from the draft. Also are you trying from an existing listing or ended? If ended is it under 60 days (they seem to work best). And of course you could try re-starting to clear out anything that might be caught in limbo somewhere messing things up.
Sometimes just weird glitches happen but everything has been okay for me (knock on wood). Good luck!