The newcomers in the Premier League are clearly suffering with Sheffield and Luton losing their games and they need to change their posture,meaning that they should understand that they are not playing in Championship anymore and that they should change tactics.
On the other hand West Ham is doing pretty well as they are the winners of Europa Conference League which was a great achievement for them last season,they are continuing where they left off and they are doing it in style in this beginning of the Premier League while soon we will see them in the group stages again of Conference League in Europe.
If it was that easy I think they would have changed their posture already!
I really think that both Luton and Sheffield are trying their best. Getting promoted into the Premier League is never going to be easy in the first few games. The intensity is definitely different and the player material is also different. The Premier League clubs have so much cash available and even teams like West Ham have a few players in the team who are really, really good. There is no free meal for any team coming up from the Championship. Every single game is like a final for them and if you start off playing against Manchester City in the first week of the season, it's essentially guaranteed that it's going to end without a point.