Hi–sorry to take so long to respond!
Are your photos themselves in Dropbox? In order to have the same project work in two different places, the paths to the photos have to be absolutely identical. If you don’t have access to the photos, they won’t show up in Tropy.
This thread doesn’t exactly answer your question, but you may find some of the information helpful in making it clear how Tropy looks for and uses your photos: Moving Photos After Import: Photos do not open in Item View