Partitioned Bitmap Index on a very large Data Warehouse partioned fact table.

We have a Bitmap index on a fieldA , very large range partitioned/month  fact table. When the user issues a query without a range specified, it

results in a full table scan even though a single value for field A is mentioned. the bitmap index does not get used?

any thoughts why ? Is it because the bitmap index is a local index and not a global one ?

thanks for suggestions / answers.