It sounds like you could be having some nock pinch. Have you checked the "tighteness" of the arrow nock between each side of the D-loop? If too tight, then while drawing back, the angle of the bowstring would be increasing and could be "pinching" the nock, causing it to lift off the rest. I like to have ~1/64"-1/32" clearance between the bottom of the nock and the lower side of the loop. You could also serve each side of the nock, inside the D-loop.
"A Nation of Sheep Breeds a Government of Wolves!"
2007/2008 Commander: 60lb/28" DL, HHA OL5519, Code Red, Choc Addiction, .400 Axis, Self-made Shoot-Thru
2010 Elite GT500: 60lb/HHA DS5500, QAD UltraRest HD Pro Series, Victory VForce HV 400
1965 Bear Kodiak Magnum Recurve 50lb, 52", Original Bear Quiver & Cedar Arrows