After right-click on a sql statement in the history and select Execution Plan History the following message occurs:
No execution plan history for the SQL statement was found
The change tracking in the agent configuration is configured for 4 times per day.
The repository tablespace runs out of space.
Increase the repository tablespace size.
After increasing the size you can start the Change Tracking manually:
Open Agent Administration | Agent | Performance Repository and click on "Track Changes Now".