RE: Diamond won't shoot fixed blades
It probably isn't the bow per se. Different riser geometry, different cam system, different in a lot of ways means you just may not be shooting it as well. I assume you're shooting the same arrows out of each bow so broadhead alignment shouldn't be an issue. I also assume you have used Easton's tuning guide and know how to broadhead tune.
Just what are your arrows doing out of the Diamond? Don't put a whole lot of stock in paper tuning. It's can tell you how your arrow is leaving the bow and should be done at several different distances, but it's not the end of the tuning process. Only the beginning.
Paper tuning cannot show you what the arrow does after it leaves the bow. Other such things as fletching drag or lack of it, FOC, and other things can enter into the picture. Too much drag (fletching) and your arrows can paper tune well, but cause groups to open up down range. Too little and they'll fly just plain awful. Too little FOC has the same effect as too much fletching. Groups open up and the farther you shoot the larger they get.
You also might try some close range (20 yards and less) bare shaft tuning and see what happens. OH yeah, and not with broadheads. It might be contact or a spine issue. As I said, different bow, different cam system.