Tiagra Rear Derailleur Won't Shift

[CheddarExpress]

So I have a trek 1000SL that I'm trying to get tuned up for ride the rockies this year. It has Sora triple shifters and everything else tiagra (9speed), and the rear derailleur won't shift up or down. When I use the shifter, it still gives and takes tension from the cable, which makes me think it must be a derailleur issue and not a shifter issue. Would getting new cables/housing fix this or do I need a new RD or a whole new gruppo?

[nfmisso]

Insufficient information to provide an answer.

Did it use to work?

Remove your rear wheel, now try shifting the RD - does it move?
Disconnect the RD cable, now manually push the RD - does it move?

[Joe_W]

Check the cables for kinks, fraying, etc. Also make sure the clamping bolt is tightened at the RD.
Also (as Nigel suggested) : Does the RD move if you loosen the cable and push it by hand?