One volunteer opportunity not displayed on Volunteer Now page

Details:

I have a CiviVolunteer project with 31 opportunities defined, each with a different role, all marked as open-ended, need=100, public=yes, enabled=yes. They all display properly on the Volunteer Now page, except one, which we'll call "Opp A", which is for "Role A". ("Opp A" needs 100, and currently has only 15, so it's not yet filled.)

Relevant research so far:

1. The project ID is 7. The option "Allow users to sign up without specifying a shift." is enabled. Calling the api VolunteerNeed.getsearchresult with {"project": 7}

   returns 31 results: "Any", plus the 30 that are showing up, but not "Opp A".

2. If I create an additional opportunity with the "Role A" role, this new one will show up on the Volunteer Now page; but "Opp A" still does not.
3. JavaScript console contains no errors.
4. "Role A" does appear in the list of options for the Role field under "FIND VOLUNTEER OPPORTUNITIES" at the top of the Volunteer Now page.
5. Except for that field, "Role A" does not appear elsewhere in the generated page source (using firebug or Chrome developer console, rather than "view source" which would only show the originally loaded source and not any opportunities).

On a hunch, I also tried toggling the "Allow users to sign up without specifying a shift." setting for this project, but that makes no difference.