There are a few different cases where your tracks can sound out of sync with each other in the postproduction. Here are some common causes and solutions.
If your tracks start off in sync and slowly get out of sync then this is a problem known as Audio Drift. This occurs due to minute differences in the sampling rate between different computers that stack up over time. Unfortunately there isn't a great fix for this aside from editing the files in an audio editor. Basically you put the tracks into your preferred editor. Make sure they start off synced. Then spot check every 5 - 10 minutes. If you notice drift, subtract slices of silence from the track that is ahead to bring them back into sync.
Late Joining Participants
Did any of your participants join the recording after it had already started? If so, Zencastr will record them but it can't line them up properly for postproduction (yet). If this is the case, you will need to take the separate tracks into your editor, line them up manually, and export your own final mixed track.
If you they go drastically out of sync quickly at a certain point during the recording, then there is a different problem. This can be caused if a participant tries to close the browser tab during a recording. Unfortunately there isn't a good work around for this problem at the moment. Please avoid doing that!
If none of the above mentioned explanations match your situation, please contact firstname.lastname@example.org or create a support request