I believe like someone mentioned above that what get most people frustrated with Martial is that they believe he can do a lot more than he is currently showing and i'm sure even his teammates think so, because you'd get several interviews from the likes of Rashford, Hererra, Shaw stating the outrageous things he does in training.
I am also certain this factors into the perseverance showed by Ole towards him because they all believe he has a high level that very few can reach. There was a period last season during the beginning of the post lockdown games to about the Chelsea FA cup semi final game where everyone thought it has finally clicked. He was working harder, terrorising defences constantly, using his strength better, linking up well with teammates. For me the Brighton match we won 3 nil was the one that most showed the version of Martial I think most want to see, although he didn't get a goal, he was just generally superb in that game, to the point he was even angry with himself and probably Ole when he was substituted because he had not got a goal. Then there was the Southampton one at OT where he was again superb, in short me and my other United supporting friends where just turning up to watch another Martial master class, some of them even believe he was the major reason we went on the winning run we did last season rather than Bruno.
But then since that FA cup semi and then the Europa semi, he has just not been the same. I had hope he would snap out of it during the beginning of the season but here we are almost halfway through the season and he still looks the disinterested version of himself. I believe if he doesn't improve before the end of the season even Ole would search for a more long term alternative, as he has been giving everything he needs to prove himself.