query tuning in 11gR2

hi I have indexes on all the colums involved in the where clause , and also all the tables are current on the stats still the query is performing a full tablescan on a very large table ST_DOCUMENT_INFO with about 55 million rows, and the query never returns.

the DBMS_SQLTUNE package is reporting as no tuning required .

can you suggest what I can do to make the query use index ?

thanks

------------------------------------------------------------------------------
Tuning Task Name : Ron_query
Tuning Task Owner : SYS
Workload Type : Single SQL Statement
Scope : COMPREHENSIVE
Time Limit(seconds): 3000
Completion Status : COMPLETED
Started at : 08/15/2012 11:51:01
Completed at : 08/15/2012 12:25:59

DBMS_SQLTUNE.REPORT_TUNING_TASK('RON_QUERY')