The performance of the big teams in the Europa League is erratic. They basically take Europa League matches lightly, which is why they don't perform well. Also the performance of the Barcelona team in the last season was very erratic. The condition of the whole party was unstable. But this season they have been able to solve most of the problems within the team. They have been performing quite consistently this season. I am quite optimistic about Barcelona's performance in the Europa League this season.
This is real problem with the Barcelona and few other big clubs in UEFA as they never love to enjoy here in Europa League and never play with their full strength even they can do things positively as well, but they feel better in their domestic leagues instead of this cup.
But, this season is surely having few changes as we are having too many legit clubs into this competition which are fighting for the title and have fair chance for the win even mostly English clubs are having tag of hot favourite because they have done good job in group stages as well but now with the arrival of Barcelona, Juventus, Ajax and Porto hopefully we will be able to have entertaining end of this season, but we need full strength performance from them.