A miss is the reason, I no longer use a scope on a crossbow. I was up in a very well located tree stand, tucked into a corner where two large properties came together, and both of them shared a fence with a game preserve. No hunting allowed on the other side, so no hunting pressure either. Lots of heavy trails, all passing through this corner junction, before heading into the other adjacent property. Simply a beautiful location.
Climbing into the stand, I had tripped and bent one of my three crossbow bolts, so to prevent confusion, I left it at the base of the tree so I wouldn't grab the wrong one.
I had does all morning. I had does all afternoon. I had does bedding down beneath me. The light is starting to fade, and a large doe heads for the fence, and I figured she was the best shot I would get that day, so I lined up the cross hairs, and just as she gathered to jump the fence, I fired.
LOTS OF NOISE! No damage! and a confused does looking in all directions but going no where!
I looked at what had caused all the noise, and saw a thumb sized branch between me and that doe, with a crossbow bolt sticking through it, and waving to beat the band!..... PARALLAX! The difference between the bolt's path and the scope's path, hid that branch from my scope. The focus was simply too tight.
I had a rare opportunity with a crossbow(a second shot), so I ever so carefully recocked the bow string, but it would not latch! I took my eyes off the deer and looked close at the crossbow, and a small metal bar was in the way of the string. I pried it up gently, and it snapped off and went clanging down the tree! The does starts to move back toward that fence, just as I got the second (and last bolt ) seated, and lined up for a quick last moment shot, ..............and watched in stupefaction as my bolt slid off the end of the bow! That little strip of spring steel was the bolt retainer, that I had caught with the bow string as I tried to quietly recock in a tree. 10 seconds after that bolt hit the tree, and the ground, there wasn't a doe in sight. And all I could do was cuss myself out, and go home.
Repair parts from Barnett took about 2 weeks to arrive, and the scope was retired before I got home. I will not use anything tighter than a reddot now with a crossbow.
The only real difference between a good tracker and a bad tracker is observation. All the same data is present for both. The rest is understanding what you are seeing.