Early 80's Bianchi with a Campy Super Record rear derailleur. It doesn't want to shift down to the smallest cog when I'm on the large chain ring. When I'm on the small front chain ring it shifts down perfectly fine.
The only thing I noticed is when on the large ring the upper jockey wheel is farther away from the freewheel due to chain tension so that the upper jockey wheel can't convince the chain to make the jump down one more cog. The chain and freewheel are both pretty new.
My only thought is a few more links added to the chain might help the situation by not stretching out the jockey wheel cage so much.