It's a combination of apparent mental deficiencies and his trademark of being a volume striker.
Harry Kane, who is overall a better player, has the same issues. He's also a volume player, but because he can also dish a cross or pass he doesn't feel the same heat.
Lukaku is a player who needs too many chances to score and many times he also needs luck because he hits centrally all the time. Under the defenders' legs, so if that doesn't work out, it won't even see the target. If he's lucky, it will take a goalward deflection which will make it harder for the keeper to stop. If he succeeds with the shot, it usually will go in as it makes a curve and keepers cannot react to it clearly due to lack of vision. If he's one on one, he almost always hits the keeper unless he had too much space and the gk makes the decision before him, which leaves an open net. This is also why PKs look easier for him. He knows how to wait for the gk to make a misstep and take advantage. In an open play situation it feels like he's afraid that someone will be close to him and just tends to fuck it up. Lautaro also suffers from this and tries to take a tough shot too soon.
If you check most of his goals in the box, they came this way.
Now, if Lukaku manages to score early on, his mental issues may go away during the game. We've seen that vs Leverkusen. Maybe Napoli awat too when Koulibaly wasn't playing. But that was it.
His main limitation is that he cannot make himself available in favorable positions, nor does he know how to evade tough defending. He needs the ball at his feet even if he's such a beast when running and unapproachable really if he puts the effort. The way he wants the ball played, he's inviting these better structured teams to eliminate him.
The best way to make full use of him in bigger games is if we have more attacking options. You need to have Lautaro, Alexis, Eriksen and Sensi on, along with at least Hakimi, and ensure that Lukaku doesn't demand the ball all the time but plays out wide to so that he attracts one defender away from their goal and makes it easier to attack from the weak side. But unfortunately as it happens with all players who thrive on the weak side, it's a "charlatan effect". It's overrated land. Guys like Sterling, Coman, Sancho, Bailey (for those who have forgotten, he was supposed to be world class as well), Carrasco etc. Obviously Lukaku is on a higher level than these guys, but unfortunately he's best used on a similar style against better teams. Which is just evidence that he's not a big time player, but obviously he has the characteristics to play for a successful team. But you cannot rely on him, he needs to have teammates to raise the level of the team. And unfortunately under Conte we have this absurd reliance on Lukaku that leads to nowhere. And it makes Lukaku look lesser when shit matters.