With indexing its a matter of the rings being too far or too close to the seat tube. A lot of indexing FD's don't have the 'throw' range to drop down into the small ring if the rings are too close to the seat tube and don't have enough outward through if the rings are to far away. Some front indexing can be 'trimmed' for chain rub. If chain line is way off the 'trim' feature wont work and it'll rub forever.