I bet you are in a dearth… Even though bees have good stores, they still want to defend what they have stored. This has been discussed extensively recently. Here are a couple of links to the most recent threads:
http://forum.honeyflow.com/t/nectar-dearth-how-good-of-a-flow-are-you-seeing/7861?u=dawn_sd
http://forum.honeyflow.com/t/suddenly-aggressive-bees/7835?u=dawn_sd