I wasn't thinking that actually but it is a version of what may be going wrong.
Personally I do see McBurnie out wide a lot in early attacks but I think he gets in position for crosses - usually outnumbered substantially by the time the ball is played across, generally hopeful crosses rather than pinpointed. Robinson is around but rarely do we get anybody else whatsoever up there in the box to distract defenders, with nobody around to pick up the second ball. Quality of crosses and no quick early ones like you say
Scooter_mcgavin. Similarly Sharp always gets in there but rarely wins a header, not that it's one of his main attributes as much as it is with McBurnie; picking up second balls is Sharp's strength though.
As for corners, I've no idea. Fleck and Norwood deliver good corners but it never works. Referees don't protect strikers but that's the same for everybody. Find ways to play in a delayed cross maybe, by which time defenders will have stopped wrestling our players.
One point that's dawned on me is the low number of headed goals in the P.L. - 27 out of 231. That's a low number by any measure and particularly when you think that when United are attacking the wide move with the cross into the box is the main ploy.