AW,
Without seeing any pics of the breaks what can I say. My start was with board bows only. Living in the south filled with good hickory and saw mills.
There is a sawmill just less than an hour from me that cuts as you would say only exotic woods (no pine), hickory, elm , maples, pecan, ash. They let me go through a pallet of wood to pick out the grains I prefer. From woods logged and, skidded and kiln dried. Only had bows break from over stress or poor design.
Could be bad wood in your instance. Too dry, grain run out, or a mixture of both. If you think so than grab a different board and go back to work. Hickory has its limits. Ive never built a molley out of a board though. I know when I want higher poundage out of a board that I have to flip a laminate over to achieve it. Normally I just rip a strip off the back(with the band saw) and flip it and glue it back on. Good luck with future endeavors.
Knapper