ORIGINAL: JOE PA
My experience has been that Slick Tricks are easier to get field point matchining flight and impact than Muzzys. Still though, I have not found Muzzys to be a really big problem. If the Muzzys were used, and had been shot several times, there is a chance that the ferrules are bent. The ferrules are a somewhat weak point with Muzzys, IMHO. The aluminum ferrule can and does bend. The blade interlocking system is the strong point of those heads I suspect the Muzzy broadheads that you have are not spinning true to the arrow shaft.
I think you hit it on the head (no pun intended). Last year and the year before, all I had to do was screw on my muzzy 4 blades and they flew just like my field points. Expecting the same this year, I had quite a surprise. My first muzzy and field points grouped well at 20 yards. My next muzzy took a dive at 30 yards and hit way low. It took me a couple of hours to find 4 muzzys that hit the same place as my field points at 30 yards. I was thinking somehow that my ferrules were damaged from hitting dirt, etc. Anyway, I'm glad I didn't take it for granted and will NEVER just screw a head on and think its good to hunt with until I've actually shot it.