"The ones that broke were ones were not backed with something."

One thing to consider is whether the bow is drying out. A board bow with an untreated surfacecould lose 2-3 percent of it's moisture within a few weeks. That's enough to break it in full draw. One thing that can be done is to apply1 to2 coatings of boiled linseedoil onto the surface of the bow. Once the oil dries, the moisture is effectively locked into the wood, and the board bow should be good for a while.