Something to keep in mind is depending on what type of target you used how did you varify that they were not opening. If it is a target where they go thru it and you can see the head on the other side they will usually be closed. When the arrow stops the blads snap back shut. That does not mean they didn't open when hitting the target. A good way to find out is to place some paper or carboard on the front or back of the target and look for the slits the blade make when going thru it. Quality mechanical blades should open every time.

Now I used NAP Spit fires the first season I hunted. I had a fairly light set up and probably shouldn't have been using mechanicals, especially the spit fires because the blades are held in by friction instead of an O-ring. I shot at an average sized deer at 18 yards and did not get a pass thru. The deer for about 400 yards and keeled over in full stride. When we field dressed it we found that it took out one lung and stopped when it hit the heart. However the blade worked as advertised. It opened fine and left a nice one inch hole in the side of the deer. I just didn't have the power to drive it thru the deer.

I promptly switched to fixed blades. Later I tried some 100 grn nail drivers. The are a small two blade mechanical with an O-ring. I put one right thru a good sized broad side doe at about 20 yards.

So for me, I will use either as long as they are the proper head for my light set up. I rarely hear of problems with them not opening unless it is a case like Arthur stated or people are re-using them and they are dirty or damaged.

Paul