That is what I thought too. I had trouble getting an F award once on an HNL-ATL-PIT award for my fiancée. It said there weren't any seats available for awards on HNL-ATL, though Delta was selling seats on that leg.

Before calling, I grabbed the URL off a news article or press release or something that described the "rule" you've talked about in a quote from a Delta representative.

When I called up (I was gold or platinum at the time), the first rep was unhelpful in a deer-in-headlights way. I asked to speak to a supervisor, and I read the quote. After about 45 minutes, it was worked out -- a space was manually opened on the flight and billed to me as "high".

The manager didn't have an explanation as to why it wouldn't easily sell.

Edit: This might have been the press release I mentioned: http://news.delta.com/index.php?s=43&item=240