When you want to sow a given seed, you need a particular type of ground/soil and particular environment to grow a particular type of harvest. If either the soil is incorrect or the environment is incorrect, a crop is either not possible or is not a great crop.
Similarly, when a teacher has certain knowledge, to make use of that knowledge a certain type of student and certain situation are necessary. If they are absent, the knowledge transfer does not take place.
Sometimes, teacher has to wait for a very long time to get a proper student or to get a proper situation. It can be tricky. Because, sometimes the student is good but the situation is not ripe. And, sometimes the situation is very suitable, but a good student is missing. A real teacher waits patiently and never gets frustrated. If the teacher has financial problems, this can become very unfortunate. A teacher should learn to make finances using whatever skills he/she has, but at the same time keeping the Yoga interest alive.
Many times, such a teacher writes his/her knowledge in books. But, those books are not substitute for the teacher. Books are written by great teachers as an inferior alternative to teaching because they could not obtain a great student.