Originally Posted by
Kontact
I looked at the specs, and the gearing you have for both crank and cassette meets Tiagra GS minimums, so you might consider going to a Shimano dealer and starting a warranty inquiry. But the answer may be "don't cross chain" or other suggestions as to when the derailleur can be expected to function correctly and when it will not.
And, of course, getting a mechanic to look at it might also be valuable in case you have missed something. Usually getting 10 speed to shift isn't difficult at all, even with a big pulley gap. The HG cassettes require very little to hook the chain up to the next cog.
Bad news, then. Warranty or further investigation won't do much because the derailleur or setup have no mechanical issues and they work: they work as they were manufactured to work, but not as Tiagra GS was advertised to work. It works within all claimed range, but having poor (hesitating) performance with smaller cassettes. That looks as a design flaw or not honest advertisement about performances of the product, but no mechanic or dealer can assist with that.
Now, I wonder... If I call it a loss, can I trust Shimano that at least 105 derailleur will work in my setup if they say so?