The way our system works is that it records your guest locally on their own computer and then uploads that audio to our servers. For the mp3 track, since it is small enough, we save the audio to our servers in pieces as it is recorded. This ensures that we have the audio on our servers even if their computer crashes or electricity goes out, etc. However, the wav files are too big to upload during the call as they would disrupt the live voip stream. So we upload the wav all at once after the recording is over.
If this upload fails for some reason, then the wav file will not have made it to our servers. This is why we can't recover a failed wav upload from our end. All audio is backed up locally on the participants computer and can be recovered by having them return to the recording page. There they can click on the track to retry the upload from their local backup.
For more details on recovering failed uploads see: How can I recover a track that didn't finish uploading?