If you think Oracle should be using hash cluster access to resolve your query and it is not doing so, then make sure the table is really hash clustered:
SELECT CLUSTER NAME FROM USER_TABLES WHERE TABLE_NAME = 'MY_TABLE'
One of three things will be returned:
No rows returned.MY_TABLE may not be a table, it may be a VIEW, or a SYNONYM over another schema's table. Check USER_VIEWS and USER_SYNONYMS to find out. If it is a view, then the SQL of the view may be too complex for your WHERE clause to be merged. If it is a synonym, try the following query instead:
SELECT CLUSTER NAME FROM DBA_TABLES WHERE OWNER = 'SCHEMA_NAME' AND TABLE_NAME = 'MY_TABLE'