Ashtanga vinyasa yoga has an obvious, linear progression. As you master a posture, you add another posture. The difficulty of postures steadily increases, as does the challenge to your endurance. Adding postures, making the practice longer and smoother, being able to accomplish more and more difficult postures: these are all signs of progress.
But sometimes you get stuck, for one reason or another. For example, personally I had a period of about six months of working to achieve basic competence in Kapotasana (catching the heels) when I didn't add anything else-stuck. I'm also just coming off twelve months of injury-stuck. There may also be times of illness, very busy life situation, held back by a teacher, etc-stuck. And in truth, Ashtanga is structured so that everyone is always "stuck" on something. Other than the necessary period of "integration" when a student has completed a full series and just practices that as a unit for six months or so, there is nearly always some posture or skill that is not ready for primetime-dropback/stand-up, for example. This aspect gives Ashtanga a certain edge that is complete intoxicating...but I digress.
All of the above-adding postures, etc-we can call "horizontal" progress. The "stuck" periods reveal the potential for "vertical" progress. Vertical progress is adding depth, it is the practice experience becoming fuller, enriched with attention to details that have otherwise been obscured. This may take any form, but is usually one of the aspects of tristana: vinyasa (movement and breath), bandha (internal attention), or drishti (gazing point). It is as if, by being "stuck," the mind can stop reaching for the next posture and suddenly you find that there is a little bit of surplus attention. After enough practices, the same routine can be done with just a little bit of attention left to explore something new...where to rest the eyes, for example, or the realization that for this particular posture, you should be counting the inhales, not the exhales.
When, as a student, you read about tristana and consider it intellectually, it seems tedious, laborious, and artificial to try to remember to pay attention to so many things at once. But simply knowing that it exists, that those elements are all to be integrated sooner or later, means that when the day arises that there is attention to spare, you are ready to smoothly take a hold of whichever aspect has been missing; it simply arises organically. This is a great example of the David Williams quote: "Before you've practiced, the theory is useless. After you've practiced, the theory is obvious."