A K2 workflow that should be running every time a document is added to the library, is only running some of the time.
The workflow is intermittently not getting fired via SharePoint Event Receiver - On Item added.
If the following error is found on SharePoint WFE event log, the issue might be related to Certificate validation failures in SharePoint:
“A certificate validation operation took X milliseconds and has exceeded the time threshold.”
If you check the SharePoint ULS log, you should find an error relating to calling the remote event receiver.
There are workarounds in the following reference that can help resolve the issue: https://support.microsoft.com/en-us/help/2625048/site-slowness-because-of-sharepoint-sts-certificate-crl-checking