Although this shouldn't happen frequently, sometimes a track can fail to upload properly if the network connection to Dropbox is spotty or if a guest leaves the recording early before the upload has a chance to finish.
In the case that a track fails to upload in a timely manner, you can reload your project page and you should see the track will have an error notification on it. Simply click the red error notification and Zencastr will finalize the track.
If the track finalization doesn't work, there is a second backup option. Every track that is recorded is saved to the computer of the person being recorded. Have the guest whose track failed navigate back to the project page of the recording and they should be able to right click the red error notification and select recover from backup. This will download the track to the guest's computer so they can send it over to you.