I've been having the same problem, I had to look into it! It was driving me nuts.
I found this on a home restoration website http://www.orhp.com/ho_quicktip.cfm
"This means there is a clog in the line between the air gap and the disposal. The hose under the sink can be removed very easily and the clog can be removed. Be sure to re-attach the hose entirely to prevent any leakage problems."
Hope this helps!