Teachers who click the Start Session link in My DyKnow are able to monitor students, but when teachers click the Start Monitor button, their classes do not show up.
To further diagnose, go to the administration console. In the Manage Data Integration section under View Import History, you should check all your recent Class Imports. There is likely a Snapshot import that is staging all your classes for deletion. This happens if you run a Snapshot import without including all classes for your institution.
To resolve this issue, you need to run an import with all your class information in the same file. Assuming no other information has changed for external course keys, you should only need to run the classes import again.
After running this import, users will be able to monitor again.