Table Index is not getting used

We have a program written with the following SQL:

         SELECT *
          FROM ztableX
          INTO TABLE li_ztableX
          FOR ALL ENTRIES IN li_ztableY
          WHERE werks          EQ li_ ztableY -werks
           AND  order           EQ li_ ztableY -order
           AND  work_cen        EQ li_ ztableY -work_cen
           AND  deleted         EQ space.

The key for the ztableX table has many fields.  We have one index created for werks, order
and work_cen.  I ran a trace of the program and found that the optimizer is not using the index.

There are values in all three fields. 

I checked the index via the dbacockpit--> Database Objects path and found that the index is valid and active.

I also reviewed SAP help and SCN and could not find any solutions.

We are using MAXDB  7.9.08.009

Do you have any advice on why it is not using the index?  What else should I look for?

Regards,

Steve