postmaster failing to shut down

Hi,
I'm new to PostgreSQL--have been using it for 3-4 months now.
I've been porting our Oracle DB and applications to PostgreSQL and I am using the latest release 7.3.

Anyway, when I do try to shutdown the postmaster with the following command:

pg_ctl -D /home/postgres/pgsql/DATA stop -m fast

I get the following:

waiting for postmaster to shut down................................................................ failed
pg_ctl: postmaster does not shut down

Any ideas why this is happening? And what I can do to avoid this?

Thank You.

Mona Gamboa