David Stewart has done over a decade of research in missionary work and retention. Some of his data are rather ugly, but we as members need to review and consider it. He shows via the statistics the main inactivity factors.
I recommend a close reading of this research book. Section I tells about the data and problems we face. Definitely a worthwhile read.
Cumorah.com - International Resources for Latter-day Saints.
Check out his entire website, as well.