Brazil's coffee farmers have been hoarding some of their crop, waiting for the right price. But buyers are testing their patience.

Coffee has been trickling out of No. 1 supplier Brazil more slowly than usual this year. The Brazilian government estimates that the annual coffee harvest that wrapped up last month is a record, but the South American country's exports are down on the year. The reason: Farmers haven't been happy with prices, which hit two-year lows in June.

But...