Are you in a humid area? When the humidity spikes here it affects my drying time greatly. If you have a dehumidifier that can help, or a dehydrator, or I've even heard of some people putting their cookies in the oven on the lowest possible setting.
I don't use egg whites so I can't say how that would affect drying. With my icing made with meringue powder, the culprit is always humidity. Good luck.