Problem in Tablespace space reclame by redusing the datafile size

Hi

in my oracle 11g rac environment with ASM one diskgroup containing users tablespace size of 130 GB was fully

used there was no any space left in any datafiles then i have trunacetd some table after taking export backup

and and import it again then around 90 GB free space is showing in users tablespace i have checked with

dba_free_space views

SELECT sum(bytes)/1024/1024/1024,file_id FROM DBA_FREE_SPACE

WHERE TABLESPACE_NAME='USERS'

group by file_id;

SUM(BYTES)/1024/1024/1024 FILE_ID

10.787109375                  2

11.5068359375                 3

11.384765625                    4

9.81097412109375            5

10.2509765625                    6

10.6123046875                      7

10.939453125                      8

10.1002197265625           9

10.515625                  10

11.4306640625                  11

but when i try to reduce the size of one datafile 1 GB only (from 13 GB to 12 GB)