There are several factors that may be involved. The Varoah mite certainly takes its toll. Then there is suspicion of Nosema disease, and perhaps new strains of Nosema. These take a toll especially in winter.

I also wonder about the systemic insecticides, however I have seen insecticidal die offs, and the bees usually make it home to die and are then tossed out front by the living bees. You will have big piles of dead bees just in front of your hives. This is not being seen, hence the "disapearing bee syndrome".

Right now the answer is, "There is no answer".