Learning tai chi requires following the teacher, monkey-see, monkey-do, which takes effort to grow proficient, and learn more quickly and accurately. Improving how you follow is part of a well-rounded practice.
It is difficult for many at first, mostly because you are trying to do two things at the same time: watching what the teacher is doing and doing it yourself. Multi-tasking. You are using the sense of sight mostly, but also that of feeling with the mind and body.
Keeping up is harder if the teacher doesn’t move slowly, or if he embellishes moves with additional, subtle changes. What the teacher is doing on the outside may seem obvious, but what he is doing on the inside may not be visible at all.
Following is a test of memory, where you try to see the next move from any point in the sequence. It’s like navigating through a strange neighborhood, or city, or wild place unfamiliar to you. You are trying to remember which way you have come so you can find your way back. You have to utilize senses in ways not used regularly. Not knowing the moves is not the issue for the beginner, really. Not knowing how to remember is.