I have found that when crafting bolts... Given enough stamina to do it 23 hours per day, you are almost guaranteed to get at least a few points. If you only do it a few hours per night, you often get nothing and sometimes 1 or few points.And now, some thoughts about "you can always raise blacksmith and artisan making knives and bolts", instead of taking Crafter. Yes, you can indeed. But it´s something that it´s very time and fatigue points consuming. Not sure about how it´s exactly managed, but assuming that it´s a 5% chance in increase for each item build, and supposing that you start building the quite useless bolts, you are going to need 10 hours and 40 fp to increase one point your Blacksmith or Artisan skill. That compares really bad with the time and fatigue expense in raising FP, Melee, Ranged, DX or ST, in a 5:1 ballpark.
Without looking at the code, I'm guessing that it is probably an individual 5-10% chance to get either artisan or blacksmithing skill increase for each bolt (you never get increase in both skills from a single bolt). It also seems to be more difficult to increase skill, the higher your current skill is.