Sometimes with everything running on your Mac it can get clustered and not be as responsive, Try and go to Applications->Utilities->Activity Monitor and Find Dock PID 167. Then click it and Quit process, This will close the dock and reopen it. And that should fix it.
Have you restarted your machine? Sometimes when I get working on lots of tasks on my Mac, it takes awhile to get any response from the dock. I save and close out of all my apps and restart the machine.