I recently came across the story of a primary school aged child who was experiencing a mental road block with subtraction. The child was otherwise reasonably comfortable with maths, but complained that what the teacher said about subtraction made no sense and that, as a result, they pretty much chatted with their friends instead of paying further attention. This may have appeared to the teacher as a lack of interest and inability to concentrate. To the parents though, knowing their child as they do, the coded message was different. It meant, "I want to understand this, and I am frustrated that I can't, so I need to make light of the fact that I tuned out of the lesson".
Have you ever been reading a textbook and found that a figure referred to in the text is located over the page? Which means you need to interrupt your train of thought to go looking for the figure, then reintegrate the information into what you were reading a moment ago. This is an example of "split-attention effect", whereby learners are required to divide their attention between two related pieces of information. The result is an increase in extrinsic cognitive load (see post 14/4/14). In other words, the way the information is presented is making you work just that little bit harder to understand the information itself. This in turn puts that little bit more pressure on your working memory, making it harder to commit the information to your long term memory.
We tend to take memory for granted, until it fails us. You often hear people say in exasperation that their mental hard drive is full and they need to delete something to take on new information. But if we look at the literature, it is not a question of exceeding our hard drive capacity, rather that we are not "saving" our mental files properly in the first place. This has important implications for learning.