If photos, videos or audio files are no longer available on the data page, the most likely scenario is that the files in question have not yet finished uploading from the app.
When a form entry is completed, the answer values are prioritized and uploaded first. The size of the answers is also relatively small and uploads quickly, even on slow network connections. However, media files such as photos, audio and video are each much larger in size and thus take longer to upload. Video files are generally the largest files (and take the longest to upload), with audio and photos usually being smaller but still significant in size - e.g., in the region of 500 kilobytes.
Mobile devices also do not allow apps to run for long in the background before either killing the apps or putting them to sleep. This is because power conservation is a key aspect of a mobile operating system - the fewer apps running, the less battery life consumed. So, generally, when the app is in the background or closed, your media file uploads may not have enough time to complete before the app is stopped. In this case, the files will remain on the device waiting for the next time the app is opened before these can continue uploading.
Additionally, if the app is forcibly closed by the user - e.g., by swiping the app off of the "open apps" view in iOS or Android - then the mobile operating system will kill the app, which in turn stops any background uploads the app may have been running.
What app users can do to ensure uploads are completed
When finished using the app, users should not kill it. Rather, they should simply push the app to the background. This is easily done by either switching to another app or by hitting the home button on the device. This will allow the app to remain open and run even when not in use.
If uploads are slow or have not successfully uploaded, mobile users can help by keeping the app open and "awake" in the foreground, or by re-opening the app when they are next on a faster network connection (e.g., an office WiFi connection). This should assist the app with having more time to upload outstanding files.
Checking progress of uploads
You can check file upload progress in two places: on the app and on the platform website.
On the app side, a count of files that are waiting to be uploaded shows on the Settings screen of the app, directly above the Force Sync and Log Out buttons. If there is no count shown, then all files have been uploaded.
On the Data area of the online platform, you will see the form entries that have been uploaded. If you mouse over each entry row, you will see the upload status of the files for that entry. This will give you a count of how many files have been uploaded out of the total files for that entry.