Martial repeatedly pressed the opponent's defense, held the ball up and got himself into promising positions. The problem is, a striker needs to be fed somehow and he isn't. Part of the problem is Rashford's poor passing from the wing and his desire to score himself. If you have wingers who enjoy scoring themselves, it's always a bit harder for the striker.
At the same time, Martial is too emotional when he's playing, as in he checks himself out of parts of the game when he is not involved.
Rashford on the other hand has more of a workhorse type of character who lacks a bit of footballing intelligence paired with some technical abilities.
Lastly though, I don't think our inconsistent attack is down to them alone. Everybody is more or less inconsistent, with Bruno perhaps being the most consistent one but also enjoying a very free role, which obviously provides him with plenty of opportunities to get involved.
I like Solksjaer, but players will always look better with a coach who has some sort of offensive strutcture and a game plan that creates spaces for them. If you have ever coached yourself, you realize how bang average a player who thrived in your system can look if used differently or poorly in another system.
Just look over at Bayern Munich, one of the top European clubs, and some players who have been there for a while to realize how good and bad some looked under different managers. Thomas Mueller is only one example of that.