Use of a Primary Index - response (15) by trustngs

Hi,
There is table defined with PI(Sale_date,store_num,item_num) and NUSI on item_nbr. Queries currently perform a full table scan, even
though the users are providing join criteria for all primary index columns.80% of the queries access one to two weeks of data. What can be done to avoid the FTS?
Anyone please advise.