iPhoto sync to iPhone 5 losing images

I have tried to rebuild my iPhoto libraries and albums (on iMac running OS 10.9.5 and iPhoto 9.5.1, with 8GB RAM), and this works initially. After a few weeks, the number of photos I have synced to my iPhone 5, running iOS8.4, gradually decrease, sometimes by a large number. What is going on?

Any suggestions, other than completely delete iPhoto and reinstall, rebuilding all albums?