I doubt there is a problem with the seed. Birds are very good at hiding that they're sick. And sick birds often hang out near feeders because they can expend less energy looking for food.
Did the dead birds appear to have any trauma (i.e., maybe a cat got them and left them near the house?) or any signs of discharge around the eyes (i.e., indicating disease)?
You could take down any feeders you have and wash them in a bleach solution (1 part bleach to 9 parts water). Rinse well with water and dry thoroughly before refilling with seed.