8-, 9- and 10-speed cassettes are all about the same overall width, so even if it was an 8-speed RD, it should work with 9- or 10-speed. Can you make the RD move over by hand (forget about the cable). Maybe it just needs a little oil at the hinges.
So...Tiagra shifters are 10-speed now? Shimano's website still lists them as 9-speed:
http://bike.shimano.com/publish/cont...e-st_road.html