Users see "Slow Down, You're posting too fast" all day

Users are only being permitted to post to the forum approximately once per day. For at least hours after their first post of the day, they get the message listed in the title. It's frustrating and confusing because normal comments are working fine.

I even tried the suggestion I found elsewhere in the forum, in which the comment flood checking is disabled completely:

remove_filter('check_comment_flood', 'check_comment_flood_db');

But this didn't change anything. My users still see the "Slow down..." message constantly. Any idea what could be causing this?