Almost all of the major brands (and many of the regionals that feed them) have had problems ramping back up to meet the summer demand. Now that it's back to school season and domestic leisure travel has eased back, it might give them a chance to catch up. It's a big puzzle and in this particular story the important is pilots qualified in specific combinations of airplane-crew position. Some of them hit the bottom of their bullpens because of extra busy holidays, major weather shutting down a hub for half a day and all the fallout from that, or cumulative effects of delays making crews timing out (which means an airplane gets stuck somewhere for a few hours or overnight).
Training is a bottleneck for a lot of airlines these last few months- mostly simulator capacity. Sims can run almost 24/7 (usually minus a maintenance period for a couple hours each day), sometimes that's what limits production. The sim instructor cadre can limit it too, those guys have a qualification matrix that involves steps in "growing" a new instructor, let alone one who can sign off checkrides. If you pull an experienced pilot off the line to be an instructor and your airline is short qualified captains in the airplane he or she is qualified in then that makes more short term pain. The instructor cadre can surge but they can also get burned out, have sick days, and otherwise be part of a complicated picture of plugging in the right qualified people to the right sim slots (so that the students aren't taking too long to finish training). Then another bottleneck can be qualified check airman availability, which has some similarities to optimizing a sim schedule.
Hindsight would have been either not give out so many buyouts and early retirements OR to restart the training departments a few months earlier. Either or both would have stocked up the bullpens much more robustly.
During covid and "quarantine," the airlines were trying to shore up market share by keeping routes open and utilizing gate slots (gate slots at some major airports are a pretty big deal with some weird rules). Sometimes that meant putting a small regional airliner on a route that would normally be flown by a narrow body airplane with twice as many seats, or larger.
This past summer the airlines shifted into building market share and they were quite ambitious about their scheduling and ticket sales volume (volume first, dollars later). Obviously this part hasn't all gone smoothly.
Hope that makes sense. Bottom-line-on-bottom: running a complicated business is complicated.