Most likely the files in question have not finished uploading from the app yet.
When a form entry is completed, the answer values are prioritised and uploaded first.
The size of the answers are also generally quite small and thus upload fast, 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 thus 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 or sleeping the apps.
This is because power conservation is a key aspect of a mobile operating system - the less 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 sufficient 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 was running.
What app users can do to ensure uploads are completed
When finished using the app, your 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.
Users should not swipe the app off the 'open apps' view of their device, rather just they should leave it in the apps list even when not in use.
If uploads are slow or 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 Wi-fi 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.
If there is no count shown, then all files have been uploaded.
On the Data area of the secure website, 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.