Candidates Not Receiving Invitation to Interviw

We have sporadic instances of candidates not receiving an invitation to interview through the ORC module. One of our example candidates was moved to the interviewing phase and state but does not have any documentation in the Messages tab showing that she ever received an invite. When contacted directly outside of ORC, the candidate confirms she did not receive any communication.

It's hard to determine a pattern because it is not occurring consistently. Has anyone else experienced this, and if so, what was the root of the issue?