Iam with the school of thought that attributes this to Loss of focus. -- my feeling is that there is a slight dimishement of "interest" after something has been repeated/accomplished. Its like been there done that. Body and mind become just slightly less interested in performing the task-- consequently slightly less focused. Kind of a interest/focus fatigue. I found for me that after shooting more than 2 arrows from the same spot, I rapidly loose interest. I suggest you move around more, and shoot less arrows in a round- This will keep your interest peaked and thus your focus. I think it's Rick Welch, (I may be wrong) who tells his students to "Admire" the target. I have found this to be an effective mental technique.--ergo-if you Admire it, it will hold your focus/attention etc. Standing there just hammering away at the same spot from the same place is kinda like boring sex- in and out, in and out, in and out. Try to make your practice a little sexier. LOL