I recently bought a lightly used, 2009 Scott CX Comp (Cyclocross) with Tiagra STI levers and a Tiagra (double) Front Derailleur. When I first got it, the front derailleur was a bit sticky when going from the large to the small ring, but I could shift between both rings.
After a few rides, I'm no longer able to shift it into the big ring. I can get the chain into the large ring, but it doesn't lock into place. It just falls back into the small ring when I let go of the shift lever. I did some experiments with cable tension by disconnecting the cable from the derailleur and holding it in my hand. When I apply very light tension, the shift lever will lock into place, but if I apply just enough tension onto the cable I get the same effect ... the lever pulls the cable but it doesn't lock into place. I tried loosening the cable on the front derailleur, but if I apply enough tension to get it into the large ring, there's so much tension that the chain goes back into the small ring. If I loosen the cable too much, it won't even go into the large ring.
I'm assuming it's either caused by an overly tight front derailleur or a problem with the front STI lever. The front derailleur seems a little stiffer than the ultegra derailleur on my road bike, but it doesn't seem outrageous. Any thoughts on something else I should check? I have an old Ultegra left shifter lying around (I bought a used pair when my road bike rear shifter broke). Should that work with a Tiagra front derailleur?