due ruote 

This is on our 2004 tandem; shifters are 9 speed Ultegra STI. Upshifting from smaller to larger chainrings is very difficult, ie. I can barely make the shift if I really shove on the lever.
If I disconnect the cable from the FD, the shifter works easily with no resistance, so I can tell there is nothing wrong with the shifter itself. I was holding/pulling on the end of the cable during this test, and it doesn't feel like there are any cable routing or housing interference issues.
So I focused on the FD itself, and eventually noticed that the arm from the pivot to the cable clamp bolt looks abnormally short. I compared it to a few other bikes, and whereas on the others that arm is about 25mm c-c, on the 105 it's only about 17mm. All I can figure is that short arm is providing inadequate leverage to move the derailleur without extraordinary force from the shifter.
Has anyone encountered this? It seems like a design flaw to me, but the 105 gear I have used in the past has all been reliable and trouble-free. Have I overlooked something?
My current plan is to try a different FD and see how that goes (naturally the ones I have on hand are the wrong clamp size). If I find a decent older Deore or XTR triple FD, will that likely solve the problem?

Retro Grouch 

Check your cable routing where the cable attaches to the derailleur. If there is a little finger in the cable route the cable is supposed to go over that little finger. If you intuitively run the cable under the finger it affects cable pull leverage.

dsbrantjr

Yes, the lever arm length is (usually) set by the distance from the pivot to the little tab, not to the clamp bolt. You might also consider changing the shift wire and housing; friction which is not apparent when pulling the wire by hand may be significant when under the spring tension from the derailleur. New cabling often makes a surprising improvement in shifting. I recommend lined housings and die-drawn inner wires; the covered inner wires make little improvement in friction, the covering often wears or peels off quickly and they can be difficult to clamp securely.

due ruote 

Good call on the little finger - I did have it routed wrong. Fixed that and installed a new cable (but not new housing as I don’t have any around). It’s improved, but still not great. Much harder to shift than the RD. I will probably look for a different FD at the co-op just to satisfy my curiosity.

KCT1986

If you try other FDs, look for ones that are designed for chainring sizes similar to your cranks. The FD-5503 triple is designed for the 52-42-30 pattern. MTB FDs in the last 20 years are designed for 42-32-22 crank.

Cable pull ratio are also said to be different between Road & MTB FD. MTB FD & shifters use more cable pull length, at least for the more current systems. Older touring systems may be different.