Did you clean it recently or change up something?
Check plumbing connections - although there's just one connection that I can recall - pump to the skimmer. Tweak the venturi maybe?
It's a common problem when they are new, but once they "break in" they are usually fine.
Anything change with the tank water? Add frags fastened with epoxy?
There are a lot of little things that can make a skimmer behave oddly.
Jenn