MSN we?re also exceptional in 15/16, but we had a dodgy run in April which unfortunately included the CL exit at Vicente and of course we allowed Madrid back in the title race but we still rallied to win the domestic double.
Luck has nothing to do with it IMO. You simply have to deliver your best version when the competition, the stage, and the opponent demand this from you. Against Atletico, our players, MSN included, had huge issues in matching their intensity. Happened in 2013-14 too. Same recipe. Go hard, with intensity, with 60k fans making noise and putting pressure, and the lazier, physically weaker, and mentally questionable Barca players fold most of the time. The Liverpool tie was lost not much differently btw.
Playing your best football and then fizzling out in April-May when the biggest competition has the deciding games is... counterproductive to say the least. Every team knows this is the time to deliver your best version. Failing that and then putting this down to luck or some random thing is just wrong imo. It's about the strength of character and professionalism, to weigh in when you must bring your absolute most competitive and fiercest version. This is what Madrid did best, they knew when the occasion demanded 100% effort and just showed up in this area big time, every time, especially against physically strong sides like Atletico, Juventus, Bayern Munich. This is where players like Ronaldo, Benzema, even Bale, Ramos, Modric, and others were at a different level compared to someone like Neymar, or Suarez.
Suarez hasn't scored in a proper away match in CL since 6 YEARS AGO, and that was a header from 1m in front of an open goal
. If that doesn't show a lack of something under pressure nothing does. And Messi was on and off post-Pep (2014/2015 and partly 2018/2019 excluded, but these are 2 years in a decade, exceptions basically), putting up insane home matches and often disappearing and walking away from home, showing he himself has questionable behavior in pressure moments when things go sour. He just gives up.
That MSN Barca had big issues dealing with bad moments. It wasn't a mature and very united team, but a fragmented collection of stars, who had their great treble season. Burned bright, but short. Ok, we can talk about winning your league, but plenty of teams win leagues every year. To be the very best, or top 2-3 you have to win your league (or fight for it until the end), and play in the final stages of CL, talking about a minimum semifinal.