postmaster does not shut down

when i grep for postmaster
ps -aef | grep postmaster

it says:
/usr/bin/postmaster -D /export/home1/sdc_image_pool/dbx -p 5432

when i try to stop postmaster doing

/usr/bin/pg_ctl -D /export/home1/sdc_image_pool/dbx/ stop
the following is displayed:
waiting for postmaster to shut down............................................................... failed
pg_ctl: postmaster does not shut down

i grepped for PGDATA

echo $PGDATA
it says undefined variable

please tell me what to do

thanks
regards
surabhi ahuja