On my mission my favorite AP was an amazing missionary. He worked hard, laid the foundation in new areas and was transferred just prior to seeing the fruits of his labors - inevitably. His entire mission, I think he actually baptized one person he taught - and that was at the end of his mission, as an AP. He didn't get frustrated because he didn't get "credit" for the people he taught who were baptized by other missionaries. He was happy simply to be involved in the process - no matter how long it took.
I believe if everyone took that outlook (including leaders and regular members) regardless of the length of time conversions took for each person, the work would flourish and retention would not be a major issue.