I'm going to jump onto the "front derailleur is not properly adjusted" bandwagon.

Here's the catch: I could teach a chimpanzee to adjust a rear derailleur but fronts can be something else. Assuming you have been fighting it for awhile, you have almost surely fubarred it beyond redemption. Here's what I'd do:

Completely disconnect the shift cable and keeping the Tiagra, pretend that you are installing that brand new, expensive, Dura Ace derailleur for the first time. Park Tool has a good set of step-by-step instructions if you think that you need them but the most important step is getting the position of the derailleur on the seat tube right. Follow all of the other steps in order because each adjustment that you make can affect the subsequent adjustments.