This may not apply to your situation, but I found that most of my problems like this were related to "general vs specific".

Many people have difficulty generalizing from specific instructions so they need help every time something looks different to them. In an extreme case, found a person unable to choose a font in the header of a word processing document because the only thing they'd ever been shown was how to choose a font in the body of the document. It's not even that they were particularly dense, it's that they'd seen so much unexpected and unexplained variation in other areas that they started assuming that everything is an isolated task with a potentially distinct set of procedures. Now that I've switched from Windows to Linux, I'm getting a better understanding of how that happens, with many applications using different hotkeys, not implementing what I think are sensible "tab ordering", etc.

Many people have difficulty going from the general to the specific without also seeing several specific examples in a variety of scenarios. That kind of thing normally requires more formalized training. If their only exposure to your knowledge is through ad-hoc help desk kinds of interactions, there will be no opportunity to put everything together.