The foll. sql has a performance problem. How do I tune this?

SELECT a.dt_tm, b.data_blob from audit_event a, audit_data_blob b WHERE a.evnt_id = b.evnt_id and serv_typ_id =5399 and dt_tm > sysdate - 1 ORDER BY a.dt_tm

The explain plan is as follows:

Full table scan on audit_data_blob and
index(range scan) on audit_event_ndx_4(dt_tm)

Even though there is an index on evnt_id column in audit_data_blob , it does not seem to use it.

Why? Please let em know.