I'm no expert by any stretch but maybe this is the problem.
Maybe your getting the same effect as taking shots with a flash outdoors at night...or in a dark place. The flash is automatically quenched when the proper exposure on the subject is reached but the background remains underexposed and dark.
Would a slower shutter speed work better here? Would you need to have 2nd curtain sync? Thus allowing the back ground to be exposed to the film first and then firing the flash to get proper exposure on the subject.
I may be way off but hopefully some smart person will help out.