I've obtained a new bike for my brother, and in tuning it after a rebuild. Sora shifters, Sora rear derailleur and RX100 front. The RX100 front derailleur doesnt seem to move sufficiently with the Sora lever.

With the front derailleur on the small ring, adjust right out to the tiniest gap between chain and inside plate on first gear, the throw on the derailleur doesnt push the chain onto the big ring. You have to hold the shifter RIGHT inside and give it 3 or 4 revolutions for it to catch and change. Then it rubs, with no adjustment.

Is it a mismatch between components??

Ideas?

Jim