Modern FD cages have highly shaped and stepped side plates based on assumptions of the chainrings they're being used with. Sometimes changing the rings is detrimental if they no longer matched the shapes of the cage.

But I suspect, that your problem may be that your 39t ring was made as an inner of a double. That means that it won't have any shift gates (cut down teeth) to start the derailing process. If that's the case, it's now a question of your confidence in your own skills with a file. Remove the ring, and compare it to the 42t you removed. Start by lining up the timing mark (if any), then use a file to make shift gates using the other as a model. This isn't that complicated, but for your first effort, go at it by degrees, mounting and testing as you go along. One place to give careful attention to the first tooth after the gate. This must shaped with the point somewhat inboard (toward the bike) compared to the rest, so it can pick up a chain that's coming from an inside angle.

Good luck.