Why ASH has no new data?

   I want to produce ASH report,but ASH has no new data.

SQL> select min(sample_time),max(sample_time) from V$ACTIVE_SESSION_HISTORY;

MIN(SAMPLE_TIME)                MAX(SAMPLE_TIME)
------------------------------ ------------------------------
25-MAR-11 01.02.28.447 AM        25-MAR-11 09.30.12.103 AM

the ASH parameter set as follow:

_ash_sampling_interval     1000
_ash_size                  1048618
_ash_enable                TRUE
_ash_disk_write_enable  TRUE
_ash_disk_filter_ratio  10
_ash_eflush_trigger   66
_ash_sample_all    FALSE
_ash_dummy_test_param  0

 

who can tell me what is the matter.