Chances are its a rotation issue, potentially /castsequence lag is delaying your casters while melee's are not experiencing the lag due to instant casts returning a successful cast within the global cool down. It may also be a synergy issue, if your druid tank is giving buffs to the DK.

Of course, its not terribly uncommon for me for my melee to out DPS the ranged. I would surmise it has something to spell clipping, which is to say that while a melee may swing up to several times per second, a 3 second cast may be clipped by the spell firing off when the mob is about to die and does so during the travel time of the spell. You also experience a faster reaction time on melee who rely just on acquiring a new target before their DPS starts again, while casters experience about a 3 second lag before their dps starts up again.

Of course, there is also an issue of Melee having an easier time of utilizing AOE moves such as whirlwind, pestilence, etc, basically giving them splash damage that you generally don't get with casters.