This is almost always caused by some sort of jQuery issue (as the menu expander uses jQuery).
Try deactivating Plugins and/or reverting to Twenty Eleven to find out where the problem lies.
Hey, it looks like my theme is wrong – I changed to Twenty Eleven and it was fine! What can I do about it so I can use mine?
Thanks for the tip!
What theme are you running?
Also, try contacting the theme author.
It’s Diary theme from Site5. I’m trying to contact them. 🙂
I managed to contact the author, we solved the problem!
What was the solution.. I am having the same issue.
The solution was posted above. -,-