Success in making converts doesn't necessarily prove the truth of a religion. If it did, then Mormonism and the faith of the Jehovah's Witnesses would have to be the truest forms of Christianity since they're growing even faster than Evangelical Protestantism.
Failure to make converts doesn't disprove a religion's claim to truth either. Only a small minority of Jesus' contemporaries accepted his message, yet this didn't refute Christ's messianic claims.
Why have Evangelicals been so successful lately in making converts? Because they're making better use of their partial truth than Catholics are making of their complete truth.
It's like the Jews and the Samaritans. God revealed himself to Israel and entrusted to it his commandments. Yet in the parable of the Good Samaritan (Lk 10:29-37), it was a Samaritan, someone regarded as defiled, who actually carried out God's commandments.
God worked through an obedient Samaritan, a man outside the salvific covenant community, rather than through a negligent priest or Levite.
Similarly, Evangelicals are doing more with less, and Catholics are doing less with more. When Catholics wake up to this reality, we'll also make converts. It's time we had, as Chesterton put it, not a Church which will move with the world, but a Church which will move the world.