Yeah once you 've shot at them, or drawn blood, forget it.. it's 10X harder now..
You may have somethign there about them having an "off day". Maybe the ones we do manage to take are "slow" and sick? that don't comprehend what's going on or are too old to fly right away and follow the others, or their brain is going and takes a while for them to process? like they have Alzheimers or something like that? :-)
All 4 times I have shot a crow, it was the 1 that stayed behind for a few secs, and did not take off like the rest of them for some reason. It wouldn't be crazy to assume maybe there was something different about those ones?