Incendiary shotgun progress

The score for the assaults incendiary shotgun challenge doesn't  seem to accumulate properly. I've got 3 kills and hit at least 15 enemies between 2 rounds and its only added up to 140 score, have to get 5000. If the people catch fire with the rounds you'd think that adds to the score also but it doesn't, it only appears to be direct hits with the pellets, which also doesn't seem to track properly.