Reclaiming unused space from datafile in oracle

Tablespace                        Used MB    Free MB   Total MB
------------------------------ ---------- ---------- ----------

 DATA_S1                               686        295        981

We have a table space named DATA_S1 which contains one data file. As we can see that around 300M space of the data file is unused. If I try to shrink the datafile by 200M by:

alter tablespace TABLESPACE_NAME datafile 'DATAFILE_NAME' resize 800M;

it gives:

ORA-03297: file contains used data beyond requested RESIZE value.

Is there anyway I can reclaim the space from datafile