Usually, the reason for this is that technology failed.

It is likely that something went wrong on the survey tool side and the status of the survey was not sent back to us.

Or there was an honesty/check validation question at the end of the survey.

Either way, when the project is finished, as part of our standard process, we always compare our status with the clients' list of completed id's. 

This allows us to manually update those that completed the survey correctly - we will notify via email all members when their survey status have been updated to completed.