Talmud Bavli Taanit 7b, 8a addresses this question widely:
ריש לקיש אמר: אם ראית תלמיד שלמודו קשה עליו כברזל
בשביל משנתו שאינה סדורה עליו שנאמר "והוא לא פנים קלקל".
מאי תקנתיה? ירבה בישיבה שנאמר "וחילים יגבר ויתרון הכשיר חכמה".
כ"ש אם משנתו סדורה לו מעיקרא...
Resh Lakish said: If you see a student to whom his studies are as hard as iron, (1) it is because his Mishna knowledge is not well ordered, as it is sai And one do not whet the edge.
What is his remedy?
Let him attend the school even more regularly, as it is said, Then must he put to more strength; but wisdom is profitable to direct.
[The latter words indicate] how much more profitable would his efforts be if he had originally systematized his studies.
Thus for example, Resh Lakish made it his practice to repeat in systematic order his studies forty times corresponding to the forty days during which the Torah was given, and only then would he come before R`Johanan.
Rabbi Adda Bar Abbahu made it his practice to repeat in systematic order his studies twenty-four times corresponding to the [twenty-four books which embody] the Torah, the Prophets and the Hagiographa, and only then would he come before Raba.
Raba said: If you see a student who finds his studies as hard as iron, it is because his teacher does not encourage him, as it is said, 'and one do not whet the edge'.
What is his remedy?
Let him seek many companions [to intercede for him with his teacher], as it is said.'Then must he put to more strength; but wisdom is profitable to direct.'
[The latter words indicate,] how much more successful he would have been had his efforts originally found favour with his teacher
שלמודו קשה עליו כברזל. שקשה לו מרוב קושיות בשביל משנתו שאינה סדורה לו ואינו זוכר מה כתיב בה ולפיכך אינו יודע לפרק אי נמי שגורסה בטעות פוטר על החיוב ומחייב על הפטור ומקשי עלה מדוכתא אחריתי והכי משמע קרא והוא לא שאינו יודע שמועתו מפני שפנים קלקל שקלקל במשנה שהיא קודם לגמרא
His learning is hard as iron, because of a multitude of kushiot, because he doesn't remember the Mishna (that is the premises of the problem). He cannot solve problems. Or he remembers wrong the Mishna, if it states chyuv or ptur and see contradiction where there isn't.