I agree with BAK....start close! hard targets can also reak havoc. but once you're on you can move back. Best I could pull off was 30, they shot well and consistent. There's bow tuning and shooter tuning, when those meet you'll be pinging bare shafts well! So to say those were crap shafts? possible but way down the list of likely's.
You mentioned arrows that flew well were marked...did you by chance change ANYTHING! diameter, paint, tapered vs un tapered? strings? anything in your shooting?
It sounds like you shoot well enough you should be able to zero in on this pretty fast.
Forgot to add...
early on when I started building arrows for others...I found (along with bowyers and scales being off), not all spine testors were created equal. It reaked havoc for awhile with a couple of guys. I ended up buying my own spine tester and testing everything myself, both what they brought in, and what I was sending out. There's just no way to get consistent without it, OR you buy arrows from one person and no one else! So what was thought to be X spine turned out to be something else! just something to ponder. It should not be this way...but I can garuntee you it is!