I wouldn't bother trying to get bareshaft arrows grouping with the others. Flight dynamics are too different without fletching. Use the bareshaft to determine correct spine, or shoot everything on the slightly stiff side. Getting broadheads grouping with field tips can be very difficult on some set-ups. Spine is very important and most people do not test spine at many different settings.

Important to check: Broadhead alignment must be perfect. Spine variance in the arrows should be very close. If not, the arrows should have been spine tuned. High FOCs can help keep broadheads from planing as much. You need to experiment with different arrow lengths, different draw weights and tip weights.

Your problem could be the drop-a-way rest, but I don't use them, so I can't help there.