This week I have been working towards updating an image service which contains our agency's RGB 8 Bit Orthoimagery holdings. I created approximately 45 source mosaics representing new missions that had been acquired. I performed QC on each one to ensure the NoData areas were transparent. In some instances I did have to rebuild the overviews. When I added the source mosaic datasets to the derived mosaic dataset, upon visual inspection, I noticed the NoData areas were no longer transparent in 2/3's of the new missions. I am struggling to identify a pattern. Has anyone had a similar experience?