Also try adjusting the snare cord/strap. The wires may need a little more slack to let them drop fully off of the head when the strainer is released.
I've run into that problem with three or four different snare drums.
If the wires are still making contact with the resonant head, even when the throwoff is released, then adjust the position of the snare wires on the reso head. Move them so that when the snares are "on," they sit a little closer to the throwoff than the butt plate. That should give you enough clearance, while still offering enough tension to hold the wires in place when the snares are engaged.