Barcelona has been performing well against big teams this season. We have seen Barcelona win by big margins in both matches against Madrid. Barcelona has also won by big margins against teams like Bayern Munich. But Barcelona has failed to win against teams like Getafe, Leganes and Las Palmas in La Liga.
It is still possible for Barcelona to occupy the top spot in La Liga. However, for this, Barcelona will have to perform well consistently. I think Hansi Flick is probably more interested in winning the Champions League title this season. That is why Barcelona is not giving full effort in La Liga matches. Hansi Flick should be more aware in La Liga matches.
If winning the Champions league title alone is his only focus, then he needs to start thinking of something else. A team that wants to win trophies will do everything in their power to win games, and not that habit of selecting games to win and games to lose. Barcelona should be taking all 3 points from teams like Getafe and Las Palmas, because gradual wins from small clubs most times gives a team the title. They really need to be decisive. If they lose to smaller teams, and win bigger teams, then it will be difficult to win the UCL.
Check their last UCL game. It was almost another night to lose to a team that they where better than (Benfica), though they found their way out. There is no big secret here. Just win your smaller games, and try to sort out bigger games.