Have read all the info already posted but the solutions appear to address all instances.  I am having trouble ONLY having the IPTC data "take" on certain .jpg files;  From some saved from one person and sent via Facebook, and now .jpg's from my own iPhone which were taken just yesterday. If I open the file in Preview and export it as a TIFF, the IPTC data saves fine.  I also just added some data to .png files and it takes fine.  There must be a setting somewhere that i have inadvertently changed.  Any thoughts?
OOPS: I FOUND THE answer! - In the Preferences, under IPTC, somehow the item "For jpegs with multiple IPTC records, read:" I had "First one" checked instead of "last one",  That seems to have fixed the problem.