Several comments:
If you shoot the same head more than once into foam, the blade retainer clip and groove get filled with foam and they won't open. Given that, the only backside of target check I would look at is the first shot with a new (or totally cleaned) head to see if the blades opened. Look at the slit in the backside of the target for a 1-1/2" dia cut versus 3/4", not the position the blades are in as momentum can snap them ahead as the friction on the shaft stops the arrow moving forward. In early 2017 I shot the same Spitfire 1-1/2" head into my Rinehart 18-1 about 50 times when I first got them. I know for sure it wasn't opening after I'd shot it a bit. Not even sure it opened the first time, but a couple of very reliable top notch bowhunters convinced me to use them due to excellent success with them. They weren't wrong...I'm still shooting them 4 seasons later.
I've never had an issue with blades failing to open in an animal, in fact you hear them open up when the arrow strikes in some cases.
They truly fly like field points from my well tuned bow.
NAP sold to the Outdoor Group and Spitfires are now made off-shore based on the packaging. The dimple plate blade retainer was thickened up from 0.004" to 0.005". The dimple is higher too....now 0.016" (new) versus 0.013" (old) on the ones I measured. This makes the blades open harder. I measured each blade on a dozen I tested and they opened with around 6 - 7#s cumulative force for the 3 blades on the recent manufactured heads versus around 4- 5#s on the original Spitfires. I measured the force using an RCBS trigger pull gage, pulling on the tip of the blade opening wing. Data was very repeatable. A bit of oil on the dimple reduced the force by an ounce or two per blade. As far as I know, the change was made to make them compatible with crossbow use. I'm still shooting my old stock right now. I did shoot a lynx last winter with a new style head and it opened up just fine. I knew I'd never see that arrow again so didn't mind the test.
Good luck!
