If you hit them where they should have died, and they didn't, I would guess that it was something that was a correctable problem. There isn't much that can go wrong with a fixed blade head, but there are a couple. One, it might not have been sharp enough. Another, is it may have been on an arrow that was not travelling straight. This can happen because of deflection, wind, low FOC, low arrow weight, out of tune bow, poorly tuned arrow or an unseen defect. None of these can be blamed on the broadhead, except a defect.

I suppose any broadhead is suseptible to a possible defect, however one of the other senerios is far more likely to have been the cause. The fact that the arrows were sticking out of the deer, suggest penetration was not good. Penetration is one of the advantages of two-blade designs, which suggests that the arrow had one of the above mentioned problems (of course I'm assuming the hit was good like you said).