Time Machine says "disk full" but not removing old backups

The subject covers it all. For weeks now my Time Machine has been telling me the backup disk is full, but it doesn't seem to be erasing old backups to make space. When I try to post this I'm asked for a community and have no idea what to choose, since "Time Machine" is not an option. Nor is "backups" or "backing up". This site is infuriating and well-night useless.