look at all your summary searches results with index=summary | stats count by search_name
try with underscore instead of spaces index=summary search_name="index_usage"
double check that you have permission to search on other indexes
look in other indexes index=* search_name="index*usage"
If you do not find any results :
test the search manually (to see the results)
check that the search ran a least one in the scheduler index=_internal source=*scheduler.log* "index*usage"
check for files stuck in the spooler $SPLUNK_HOME/var/run/spool/splunk (temporary csv files of the results ready to be indexed to the summary index, if they are not deleted after indexing, then they were not indexed.)
check that you can actually index things (in case of complex forwarding setup)