Which of the following troubleshooting steps did the administrator miss?

A SaaS customer reports that no one can access the application portal. After some troubleshooting, the cloud administrator finds a problem and provides a remedy by restarting the cluster. After documenting the solution, the support case is closed. Multiple tenants lose access to their hosts the following day.
Which of the following troubleshooting steps did the administrator miss?
A. Identify the problem.
B. Establish a theory.
C. Verify full system functionality.
D. Test the theory.
E. Establish a plan of action.