Sometimes they come back.
I am a convert to Orthodoxy and was Chrismated into an OCA church in 1991. I was very active in the church and as I later moved from state to state and town to town, I continued to attend services regularly in the cities I moved to or where there was an Orthodox Church nearest to me, but eventually fell away. I blame only my own sorry sinful state of being for allowing myself to fall away from the true faith. Many years have gone by since I was in regular attendance but I always considered myself Orthodox and never even thought of going anywhere else. After all, how could anyone find any other form of worship equal to what we have been handed down to us through the Church; the Divine Liturgy, The Sacraments, the teaching of the Church, the Tradition, and the continuity with the Church Fathers. Thanks and Praise be to God, I am finally making my way back and it feels great to be back!
Again, I blame only myself for being away from God and the Church, but as a convert to Orthodoxy who became Orthodox in an all English parish made up of about 50% converts and 50% cradle Orthodox, as I moved around and went to other Churches I found language to be a barrier. I've been in OCA Churches that were 100% Old Church Slavonic, and Greek Churches that were either all Greek, or mostly Greek, and I remember one time going for coffee hour after the Liturgy in a Greek Church once where a little old lady came up to me and asked: "Are you Greek?" - No; "Are you married to one?" - No; "Then why are you here?" I know other people who have had almost this exact same experience in Greek parishes.
I speak only for myself, but I wonder if experiences like this may be a cause for a turn off (or falling away) for others? Language is an issue that needs to be addressed in the US. I understand the need that used to exist to use Church Slavonic or Kyrie Greek because the parish was established by immigrants from Russia, Greece, Serbia, or where ever, but the periods of heavy immigration from Russia and Greece are over and most are now 2nd, 3rd, and even 4th generation from their immigrant ancestors. If the Church is going to thrive in the United States, it must (in my opinion) address the issue of language and the language it needs to use here is English.
What do our Church Fathers say about language in the Church? The Apostles went out and spoke the Good News of Christ to the multitudes in their own language with the Grace of the Holy Spirit as recorded in Acts. Saints Cyril and Methodius didn't force the Rus to learn Greek or Latin. They created the Cyrillic alphabet and translated everything for them. Likewise I believe (please correct me if I'm wrong), Saint Tikhon of Alaska translated to the native Alaskan Indians into their language.
So I wonder if the reason some converts may fall away are due to this kind of stuff?
I love the Church and hope that all Orthodox in America can one day call themselves Orthodox of America and not just Orthodox in America and that we will find unity amongst ourselves so that we're not Greek Orthodox, or Russian Orthodox, or Serbian Orthodox, or what have you, but instead we can call ourselves just simply Orthodox Christians.