While you were away, they did a patch where "good loot" would stop dropping if you farmed them. For example, 2 champions/elites on DK near and in the Dark Temple. Plus the scientist and 2 adds. They had some very good and rare loot drops. But after the 3rd or so kill of that mob, loot quality diminished. So you had to wait or log out to reset it.
I assume this is the case unless you tell us where you are farming.