Problems shutting down Postmaster

I need to add a system to our postgres pg_hba.conf file; the main server went down a few days ago, and this system was mistakenly left off the list of allowed systems when the server was brought back up. (Older version of the .conf file; I managed to accidentally delete the more current one while trying to copy it for safekeeping. Proves what an idiot I am...)

Someone else added the IP address for the other system to the pg_hba.conf file later, but since we had already started postgres by then, it didn't take. And now, for some reason, postgres doesn't seem to want to let me shut down. I keep getting the following error when trying to shut the postmaster down:

./pg_ctl stop -D /usr/local/pgsql/data/
waiting for postmaster to shut down............................................................... failed
pg_ctl: postmaster does not shut down

I am doing this as the postgres user, so I'm not sure what else I might be doing wrong. Does anyone have any kind of suggestions about how to solve this? Alternately, would someone please let me know of other ways to shut down the postmaster so we can restart it with the updated .conf file?

Thank you for your consideration.

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