unable to increase retention period of backup image

Hi Fellas

master , media, client OS linux  NBU ver 7.1.0.2 , backup goes to data domain box model number DD670 -- copy 1, then duplication to tape. copy 2 n copy 3. customer requested to increase retention period of backup image which is on DD box. 

first used bpimagelist to determine images for particular client.

then used bpexpdate -recalculate -backupid <servername_134665620821> -copy 1 -d 09/02/2013 13:00:00 -force .it executed successfully. but problem is when i tried to cross verify after 4 hrs that retention period increased on backup image or not it still shows the same retention period.

could not understand why it is so even though bpexpdate was successfully executed.

???????????