Get down on your hands and knees with a torch and make sure that the male plug in the dock lines up with the female socket on the bottom of your ipod. my wife had this
problem and her skinny ipod was going under the male plug as there was no plastic adaptor in place for her skinny little ipod.
Also I think the male plug in the dock could get bent...........
EDIT.
Does your ipod work in another dock, maybe try it out at your local Commet or Currys, they have loads of hi-fi stuff with built in docks that you could try.