I’ve often seen that sync operations take a while and seem fragile. Yesterday, I performed a sync on my Android phone at a place that had wifi access, though perhaps not as stable as at my house. The app got stuck in a sync operation. I forced the app to stop, and then restarted it, but every time it started, it wouldn’t get out of the sync loop – unless I turned off internet access. But then it proved impossible to sync. I eventually had to delete and reinstall the app, thereby losing all the sentences I had done that day.
Have you considered any of the following?
(1) Performing incremental syncs. If you could fully upload 10 sentences at a time, and make the upload of those 10 sentences a fully atomic operation, then a maximum of 10 sentences would be lost if something went wrong. I wouldn’t mind that.
(2) Providing a cancel button. Again, if it interrupted an atomic upload of 10 sentences, I wouldn’t mind.
Note that the symptoms were similar to the ones mentioned in this post from a year ago:
but the error dialog was not displayed.