Then I would say your expectations are unrealistic and we have pretty different definitions of sitter, you seem to act as if the GK doesn't exist in 1v1; they were chances, but not sitters. One was from a tight angle with the keeper covering his position, another was a rush from GK after the first touch where there's not much space to score at that point, and then we have a shot from outside the area he tries to curl in. I can't remember three 1v1 that are sitters in any of those. They're still at least one chance out of those I feel Martial should have put away, with maybe a better first touch from Pogba's pass and quick strike when the GK rushed him in the surface. He should have done better with that one, the others I don't feel were easy chances.
Either way, this can only light an even bigger fire in his belly, I thought the Copenhagen game would have done that but he came up short in the end again. For me, it's still an anomaly more than a trend, and I'm not worried about him hitting 20 league goals next season provided he's healthy for most of the season. If Sancho comes, 25 league goals is very realistic if he plays most games.