It is complicated to answer but there are several factors which contribute toward the difficulty of learning how to solve problems. A person's motivation (anxiety) and poor understanding of the how and why of the material. But there are more internal factors as well, mental and metal learning handicaps. For example, Dyscalculia is a common condition where individuals have issues with making basic calculations and using those numbers the way their peers do. (Dyslexia, ADD/ADHAD, Dysgraphia, and Dyspraxia are other examples.
