The money used to buy players contribute to how the player is be received at the new club, this isn't only a Chelsea thing as other clubs are experiencing same results. Chelsea too has players they overbought that are flopping and also players that were bought for their worth and doing well for the club. When a player is been overbought, the fans expect him to start proving his worth immediately and that can disorganized the players because he's joining a new club and need some readjustment period but doesn't have that because of what the fans and coaches are expecting from him. Low price put you under the radar and even when you're not performing well people will give you a break because you didn't cost them much.
We don't have forward that can be relied on and thats has been our problem from last season but the coaches and club owners has refused to recognized that. They thought they have solved that problem when they signed Nkunku but forgot to do their due diligence about his injuries. Nkunku doesn't play a complete season without having some injury crises and that's what has begun without the season starting properly. Sterling can't be relied on every game because he isn't a consistent player. The only reason he was doing so well at Manchester city was because he was surrounded with world class player with lots of experience and a worthy manager but at Chelsea things hasn't been as expected.