Bayern Munich and Barcelona are two differen team with different work load. When Lewandowski was in Bayern Munich I don't think the pressure he was facing there can be compared to what is in Barcelona. Bayern Munich used to be the strongest team in the Bundesliga league, and with no tough competitors. If Lewandowski is not doing more in Barcelona I won't blame him because Barcelona is competing with more tough teams than Bayern Munich.
I beg to differ though, Bayern Munich has tough competitors, the readiness of other teams and how they respond to every match against Bayern is subjective to people, just the way people prefer to watch Premier League and Champions League than any other leagues and of course, players also influence the viewers and fans base, just imagine the number of players who became PSG fans because of Messi and Ronaldo when he moves back to Manchester United from Juventus.
The problem with most of the clubs is that they don't even know the potential of these players until they sell them, that's when players switch into beasts that they haven't displayed in their previous clubs, take a look at Lewandoski's moves from Bayern Munich to Barcelona, Haaland from Dortmund to Manchester City.