Are you certain there is a gap left between the clamp and the surface it meets? If no gap, then you might be over torquing the bolts and fracturing them just enough to think the seats secure and then they break when normal ride forces are encountered.
I used to similarly break bolts for seat post clamps trying to tighten them enough to keep the seat from falling. Either the slot in the steel tube of my bike would be closed up under the clamp or the clamp would be fully closed. Thankfully I knew about them breaking before riding.