Oracle Database Discussions

Hi,
on 10g R2, in OEM (DB control) For Top SQL: Application I have :

 Data for the chosen interval is not available. 
 

Why ? Should I enable something or set a parameter ?

I have this setting :

SQL> show parameter STATISTICS_LEVEL

NAME                                 TYPE        VALUE
------------------------------------ ----------- ---------
statistics_level                     string      TYPICAL

Thank you.