[embee]

Is the gear lever free on the pivot and are the rod ends free and lubricated. It was quite a common problem on the early SV bikes for the lever to seize up on the pivot pin, IIRC there is a plastic bushing in there which can break up (may be wrong, can't recall exactly and it's a long time since I was in there on mine).
If a stiff lever pivot is preventing the selector mechanism from returning all the way to the centre even by a small amount it cannot catch the next downshift.