Metadata not being written to files

[Alan Seal]

I am creating titles and captions for photos. In one album there are 
captions and titles for every image. I have tried Item Write Metadata to 
file for individual photos and also for the whole album. Most get the 
data written to the file but the last three images do not - caption and 
title are blank.

Why might this happen. The images seem to be the same.

Alan