Joe, thanks so much for responding. So you think I might also be underdeveloping? It's something I should experiment with, I suppose. I'll experiment with a "proper" light source for preflashing. I like your ideas.
Ralph, I'm intrigued by the yellow filter suggestion. Silly question maybe, but do you just hold the filter in front of the pinhole? The results look good.
For pinhole photography, it's held from underneath on the lens plate by a lump of blue-tack. But I also make paper-negatives with lens-based photography. I rate the filter/paper combo as ASA 3 for MG-IV paper. It gives me a beautiful tonality and easy-to-print negatives.