The thing is he is starting to practice two-voice exercises and is finding that he is making many more mistakes than in single voice exercises. So he is wondering if he is doing something wrong.
I wrote to him that he needs an extremely slow tempo when doing these two-part exercises because the texture is begining to look much more polyphonic. Normally people understand the necessity of the slow practice and start my course really slow which is a good thing.
As the single voice exercises progress, often they are starting to feel that they are advancing their skills and so they make very few mistakes. So after some weeks into this course they can sight-read one voice melody fairly well.
And then two-part exercises begin to appear and it becomes really hard for some of my students. I think that many of them even didn't notice their tremendous progress and they naturally began to play a little faster with single voice exercises.
And so they started two-part exercises with the same mindset and tempo which is obviously too fast. So the only thing they need to do in order for the exercises to begin to feel quite easy again is to slow down the tempo so much that they could play the exercises rhythmically and melodically correct. This means extremely slow but rhythmically stable practice.
If you ever encounter such difficulties when learning to sight-read on the organ, try to apply this tip and you might be surprised and how easy the exercise or the piece might become for you.