Oracle Database Discussions

Hi
I am working on index Tablespace which is having around 20GB of Free Space,
But when I tried to release space by resizing the datafiles it is giving me following error.

ALTER DATABASE DATAFILE '/d001/pistss_2006_01.dbf' RESIZE 3000M;
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value


This tablespace only contain indexes in it and the tablespace has been created as segment space management Manual.
Please suggest if any one has faced such issue.