Tend to agree with LM most probable cause lots of failed firings would likely have been the aircraft sys. The missiles themselves were pretty reliable and pretty easy to keep in good shape. I spent some time at MPC in the late 60's and then visited with a squadron in the early 70's for a couple of annual visits. There was a pretty rigidly enforced "random sampling" for live warhead firings, principally to evaluate missile serviceability. Missiles were sometimes fitted with a Flash Warhead (Reduced Range Safety Trace) and occasionally a Telemetry Unit; these though were considered tampered with and did not qualify to be assessed as a random shot. Any failures were investigated in depth. Generally though, in those days, success rates were high. Good times, the esprit de corps in the Lightning force in those days was, IMHO, second to none in the Air Force for both drivers and maintainers.