I remember Romelu Lukaku being a great striker too until he moved to Chelsea.... After that his performance got worse... Even now that he's playing at Inter, he can't get back to his best past results... It could also happen to Kane if he changes teams and leagues... I guess it's all about habit and age... If you're not young, it's harder for you to change your familiar surroundings...
Romelu Lukaku lost very good skills when playing for Chelsea whereas before he was a pretty great player for a very high price.
Maybe it's because Chelsea are experiencing quite a significant decline in terms of play, so it has indirectly affected Romelu Lukaku's performance as well.
Romelu Lukaku's return to Inter also cannot guarantee he will regain his playing skills because he has already experienced a decline in playing performance and of course it can take a little longer to recover.