Change based alerts were setup correctly and previously working. Then some of the Change based alerts "Subjects" stopped getting generated while others continued to be generated and delivered via email. It was discovered that if you toggled the "Enabled" check mark under | Organization administration | Setup | Alerts | Alert rules | for the specific Subject Alert rule that was not working, it would start generating alerts again, and produce the emails that you would expect in the "E-mail sending status" in System administration.
Question: How can I identify when these alerts stop working and also how can I troubleshoot as to why they have stopped generating?