As you pointed out, since Pep, we haven't pressed like we used to in midfield which kills us in defence. In the last few years I put that down to Pep leaving and Tito's absence but it actually could be that the boys just don't have the legs for it anymore.
...
For sure it's because players are slowing down. Barca is slower at nearly every position than they were in their prime around 2010, except for Alba and Neymar. Even the relatively young Messi, Busquets, and Pique are slower, because of overuse, I would say. Slow in speed, quickness, with the ball, and without, covering range, everything. That's why teams get through their initial press so much easier then they used to.
Pressing is not just about stamina. It's about covering space, and Barca players don't cover as much as they could. A pass that Xavi could intercept with one lunge a few years ago he can't get to today. It's also about 1 on 1 defending. Xavi and Iniesta were lethal 1 on 1 before - today, they get beaten. So they need extra support, which means less players to cover the passing lanes, and the press is weak.
The adjustment of course is to press smarter, and this is the one thing I have not seen Tata able to implement so far, but I think it may take months. Smarter means better choices of when to press and when not to press. For the past year, Barca have been swarming the ball mindlessly, gambling too much. Once the ball goes past, they're in trouble. They should more systematically close the channels then attack.
The other factor in successful pressing that is often overlooked is potency in attack. Last year, good teams had no fear that Barca would score quickly on them. They knew that as long they were solid defensively, Barca would pass the ball around but not score. So there was no panic when they got the ball and launched their counterattacks. The last match was a good example. Sevilla had belief to the last and carefully launched counterattack after counterattack. A potent attack would cause panic which would help the pressing regain the ball.
Everything is linked with everything.