Did you move the original photos by any chance? The thumbnails are created and cached by Tropy; when loaded into the item view, the original photos are used, so if they were moved or renamed, you'd have to consolidate the photo library. You can check if the photo was moved by right clicking on a photo and opening the containing folder.
This might also be relevant when moving your project to a new computer (other thread): currently Tropy allows you to consolidate only one photo at a time if it has moved. In most cases, however, it's fairly easy to update all the photo paths at once by updating the project file directly. See the docs for further information.