First check is if Mirroring Exists on a server. Checking for mirroring_state that exists in sys.database_mirroring. If none exist, alert will not fire. Designed to avoid false positive alerts when Mirroring is not setup.
Large log send or redo queues can be an indicator of a system or network bottleneck. This condition will be triggered if either queue goes over 3MB.
Use the Performance Advisor Dashboard to troubleshoot.