I had a problem where one gallery would work, but two others would show the first 5 thumbnails only and the first large image would just be constantly "loading....".
Seeing one picture's IPTC data not display in the gallery that worked, I figured out the problem.
There seems to be an issue with dashes (-)...possibly special characters too...within the IPTC data for that problem, but for the filename of the pictures for the loading problem.
I deleted them all and everything works great. It seems underscores, numbers and spaces are all ok as well.