1) Organists feel a tremendous amount of fear when playing without scores. This is especially so true when they play in a public setting, such as an organ recital or a church service. A person can feel very insecure and frustrated because he/she may experience a real memory loss at any given moment during the performance.
2) When learning to play from memory this process can be very frustrating because no matter how hard you are trying to memorize, no matter how long you play, you still can't play your piece without score fluently, if you are not doing it systematically step by step.
3) Very often people try to memorize a piece of organ music playing it over and over again. This practice is too complex because the piece itself can be quite long and no matter how many times you repeat this material, you may be forgetting many important details.
4) Incorrect practicing tempo can be a deciding factor whether or not your memorization efforts will be successful. You see, if you play the piece too fast, then you will not be able to use your mind at the same speed as your fingers or feet. On the contrary - when memorizing, your mind always goes ahead of fingers and feet.
5) Playing the entire polyphonic texture at once and trying to commit it all to memory is also too challenging. In pieces, such as fugues, chorale preludes or fantasias of the Baroque period every voice is quite independent. Therefore, sometimes it helps to memorize separate voices and their combinations first.
6) Trying to memorize a piece of organ music before actually learning to play it is not a good idea, either. Memorization is one of the last steps you should take towards a complete mastery, not the first.
7) Forgetting previously memorized fragments is often a huge difficulty some organists have to overcome. Let's say you are learning some new fragment every day which of course is a very good idea. However, you should also systematically repeat the episodes and fragments that you memorized the day before and earlier.
By the way, do you want to learn my special powerful techniques which help me to memorize any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.