Originally Posted by
oldbobcat
The first question is one of semantics. What do you mean by "catch?" Is the shift lever not holding tension on the cable after you ratchet it to the next click, or is the chain not being pulled far enough to "catch" onto the next ring? The first would be a problem with the shifter itself. The second would be a problem with the derailleur or the tension of the cable pulling on it.
A third possibility. The shift lever was not catching on the ratchet advance mechanism internal to the shifter so it wasn't pulling the cable at all. All is good now. A few squirts of WD40 loosened it, and some fresh lube put in its place and it all works again. It holds and shifts pretty cleanly for an indexed front triple.