Lukaku is a good striker, but he has two big problems that are holding back his game. Technical ability and temperament. Technical ability has been done to death already, so let's talk about the temperament issue.
The way I see it, Lukaku struggles to keep calm during games. In other words, he gets frustrated too easily. If we look at the Liverpool game, he had one big chance to win the game. Now I know that all strikers miss chances. But its also true that some strikers score more crucial goals than others. And in the Liverpool game we could see from minute one that he was frustrated. Its no wonder that when the chance finally came, he couldn't keep a cool head and ultimately missed it.
In other games, from what I've seen, Lukaku tends to over-react when teammates misplace a pass or the through-pass isn't good enough. He usually looks very frustrated, more than he needs to be. Even in the Tottenham game, the moment his header hit the bar, it was like he switched-off for a while. He moped about for at least 5s and looked away from the ball. He should have been getting into position immediately because there's a chance a teammate could send the ball back into the box.
If he wants to start scoring in big games, he needs to accept that chances will be few and far between. He needs to learn to keep a cool head so that when the chance comes, he is in the best frame of mind to take it.