1. Negative self-concept: we think of ourselves forgetting things.
2. We have not learned the material well.
If something is to be retained, it must be correctly, clearly and forcibly impressed on the mind. We must give it the necessary attention and interest. Self-questioning and spaced or periodic reviews are essential.
3. Psychological reasons: defensive forgetting
Generally, unpleasant things are remembered better than pleasant things (especially by pessimists), and both pleasant and unpleasant things are remembered better than materials we are indifferent to.
Memories fade away rapidly when not reviewed or used. But "forgotten" material can be relearned in less time than is required for the original learning, even after many years' disuse. Even material that we do not relearn has undoubtedly been transformed into attitudes and values that form the foundations of our judgment. Education pays in spite of all the details that are forgotten.
5. Interference due to emotional problems, anxieties, distractions, intense concentration on something else, and intellectual interference
Intellectual interference or mental overcrowding can be minimized if we reflect on our reading and experiences, understand them, clarify them, associate, and organize them so they will not interfere with each other. We must avoid cramming and overcrowding our learning hours with unorganized material. There is more interference between two similar subjects than between two unlike subjects. (Follow study of history with chemistry rather than English history or literature.) Furthermore, there should be rest periods at intervals to allow the brain to lie fallow. Continuous undifferentiated activities apparently fight for a place in the memory. Since we cannot be awake without thinking, it should follow that there is more loss of memory for learned material when one is awake than when one is sleeping. (So study and then sleep.)
6. Changed Cues
Recall what happens on tests. You studied the material one way, the test question was presented in another (cues were changed). If you learn material with too great dependence upon the phraseology of the textbook, you may be at a loss to remember some of the material if you cannot recall the exact wording of the text. As with your outlining assignments, restate or rephrase the ideas in your own words to insure remembering.
7. Lack of Attention and Effort
The art of memory is the art of attention-attending to the material WHOLLY. Moreover, there must be effort to remember.