Sometimes, depending on what bolt you used, the interface or hole you hex wrench sits in can be quite shallow, and thus if the tool, whilst feeling like it is in all the way, is not, you can sheer the bolt head, or if their is dirt in there then you have the same problem of the tool not getting enough grip and rounding out or sheering the head off. Also general wear and tear can be attributable to broken bolts or indeed over torquing....SS Jerry said:
Had no such problem with either my Ergo1s or dragons.