There are many teams that perform well against strong teams and perform poorly against less strong teams. Sometimes strong teams do not know the past analysis of weak teams and their players in detail so during training, the match situation goes against them due to not knowing the strengths of the opposing team players. If they were aware of other strong teams and their players in advance and practices their strategies to counter them in advance it would be easier to play against them. The same situation was seen in the case of Barcelona, they performed poorly against Rayo Vallecano. But despite being a weak team played very well because the Vallecano players were aware of Barca strengths and went to the field after studying the players of the opposing team.
I doubt that Barcelona didn’t study Rayo Vallecano before this match, but that doesn’t change the fact that Rayo Vallecano have an excellent defense, and they should have expected that they would defend. But Barcelona didn’t try to force the game until the moment Rayo Vallecano equalized, and after that they were unlucky because they couldn’t adjust for the rest of the match. So in this game, the draw became possible only thanks to the hosts’ defense.
Flick need to work more on the team for stable their performance. Barcelona scored two goals before Rayo, I don't see any reason why Barcelona will be so lose that they allowed Rayo to break into their defense easily and equalized the goals when Barca knows that Rayo has a solid defense and would be a problem for them to breakthrough. Barca messed up.