A protection plan's scheduled task can only run one instance of the plan at a time. If the plan is already running when the next scheduled runtime is met, then the task will return a status that the plan is already running.
You may see this in Archive Manager when the remote computer is selected in the left pane and protection plans is selected in the right pane. That status may also appear in the Result column of the 24 hour checkup report. See image below:
The checkup report may also indicate a Result error 8004131F, which translates to 'an instance of this task is already running'.
The plan should continue to run to completion, and then start again at the next scheduled runtime. In this case, you might want to adjust the schedule so it does not run as frequently. However, in some cases the plan process on the computer running the plan may become hung or stuck. Because the plan's processes must end before the plan can start again, the plan will not start again in this state. Check the Start column of the checkup report daily to see if the same start date and time is indicating a hung plan.
To prepare the hung plan to start again, log onto the remote computer running the plan (or the Archive Manager server if it is a local plan). Open Task Manager. You can right click on the Task Bar at the bottom of the screen and select Task Manager from the pop up menu. Go to the details tab and end all instances of aiq.exe, aiqremote.exe, aiqremote64.exe, ArchiveIQLaunch.exe, and SQLPlanLaunch.exe associated with the plan. You can right click on the process and then select 'end process' from the pop up menu. The plan will then be set to start again.
Note: Local plans run using the aiq.exe binary. Aiq.exe is also used to run store tasks. Take care to only end the aiq.exe processes for the appropriate plan. Open Task Manager, details tab, and add the command line column to determine the task or plan using the process. You can right click any column header and click 'select columns' from the pop up menu to add the column. If multiple plans are configured on the server in question, take care not to end the processes for the other plan, if it is currently running and is not hung. You can also add columns to Task Manager on the details tab to monitor IO read bytes and IO write bytes for these processes to see whether they increment.