Barca won 3-0 at the Nou Camp, with Dani Olmo, Raphinha and Lamine Yamal the goalscorers.
Afterwards, Flick said: "There are a lot of matches and trips, and that's no excuse, but it's taken us a little longer to reach our best level.
"We did it in the second half.
"They pressed very high up the pitch and we didn't play at our best in the first half. We improved a lot in the second half, we scored quickly and that was good.
"We have one game after another, a lot of travel... We hardly have any days off and we have to take that into account. I'm aware that it's our job. The team always gives its best."
On Joao Cancelo, who was changed in the second-half, Flick also said: "He's just arrived, but we see his quality in training. He's also done very well in some situations during the game, but he needs to adapt to our philosophy. We're working on it.
"He needs to adapt a bit more. I also wanted to give Balde a rest, manage his playing time. He's probably more of an attacking player than a full-back. He has room for improvement, but he's already taken the first step. He started and played very well."