You could try making a new plist for the Dock:
HD/Users/YourUserName/Library/Preferences/com.apple.dock.plist
or
HD/Users/YourUserName/Library/Preferences/com.apple.dock.db
one at a time if you want to know which one fixes it (if it does):
Move the plist file to the Desktop
Restart the computer. This makes a new plist, set to default values, which means anything you've messed with reverts to "out of the box".
If it didn't help, you can put the plist back. This will overwrite the new default version.