Hi,
first check if the secondary index is really used in the execution plan for the query. You can also use HINT clause to be sure, that secondary index is in use:
SELECT [..] FROM [..]
WHERE [..] GROUP BY [..] HAVING [..]
ORDER BY [..]
%_HINTS <selektor> '<text>' <selektor> '<text>' [..] .
More about hints you can find in OSS note 12935. Order of fields in the where clause and also all used in where fields should be included in index.
Hope it will help.
BR
Paul
