From my experience as a guitar teacher I discovered that most people expect their progress on guitar to be constant and uninterrupted. The reality is there will be periods of stagnation even if you are practicing daily. Why this happens I cannot explain. All I know is expecting uninterrupted progress tends to lead to disillusionment. Most people quit at the first plateau. Once I understood this and began explaining it to my students I noticed my student retention rate improved dramatically. This idea is of course not exclusive to guitar. It applies to any skill that takes time to learn including the skill of teaching guitar. Apologies to my students in those early years.