iOS not all photos sync with Nextcloud

A user on my family Nextcloud server is having issues syncing pictures. If you open the app and go to the “Photos” tab, all the images show up there. However, looking at the directory where pictures are supposed to be synchronized to does not have all the images. Recently I did “Upload the whole camera roll” to ensure I got everything, and now days later some new photos have been uploaded and others haven’t. What could cause some pictures to not upload to the server?