Badri,
As what nbaig says, he's right, I'm just giving some points to be sure what u'r doing.
1. Check the datafiles associated with the TBS using v$Tablespace
2. Be sure that the TBS is made offline NORMALLY only and not immediate or temporary
3. For faster results issue a checkpoint befre taking the TBS offline.
4. Remember, more the time it takes to copy a datafile, more pefomance overhead is added. So make the copying of datafiles fast.
5. See to that, there is adequate space in the archive dest directory.

Regards
Ganesh .R