I have had fits with this the last year after some form changes. A lot of things made temporary improvements, but I think it is mostly in the release if you eliminate the arrow spine, and nock height. I put white electrical tape on the edge of the self so I could quickly see if one thing or another stopped it, and I could keep tabs on it when I was not hitting to make sure I didn’t start. You can see rubs easy with the tape on sometimes just one arrow.
Arrow spine has an effect, but can be misleading. A weak arrow can mask it when a correct arrow will seem to cause it. Slight changes to spine or draw length can get the timing just right so you miss the shelf until you change something a little.
String torque vertical can change the limb timing to cause the hit. While grip and other things can also cause it, they can also mask the root cause when you play with those variables.
Check out the thread about finger blister or sore ring finger and to see if this is your problem. Torquing the string side to side and up and down with a getting off the string late with the ring finger can do it for sure. I had some torque, but mostly late off with the ring finger causing a lot of my problem. I think that throws the shaft wide and then back into the shelf before the arrow clears. Late off with the ring finger also causes sort of a three under or worse limb timing issue that drops the back of the arrow lower than it should. Funny thing is I can shoot three under no problem. I get off the string much easier for some reason. Must be string angle, pull angle, or something with the fingers being all nice and close makes the difference for me.