What are some ways to address inaccessibility of to-be-learned material for research? Throughout my PhD I learned that there are lots of stumbling blocks that prevented me from getting to my goal of solving some problem. One of them is that some of the things I needed to learn were inaccessible and posed a number of undesirable difficulties. For example, sometimes papers I need to refer to are written in a language I don't understand. Some books lack notation indices, with the result that the book was difficult to follow. Sometimes the papers are hard for non-experts to understand, and I would waste a lot of time wondering what an author might mean. Sometimes nobody at my university knew the answers to my questions, even though they are on "well-known" facts (well-known to the experts but not to the non-experts).  Sometimes I would be given incorrect information from an expert. In many of these cases, I didn't get my questions answered until some expository books on the subject came out. In other words, a great deal of inefficiency can arise that cause unnecessary and unhelpful delays in establishing a publishable result. I think these difficulties are causing delays in my own research. Keeping everything else the same, another researcher who doesn't have to encounter all these blocks would arrive at results faster and would publish more papers, and would be seen as more "exceptional."

How do researchers deal with these issues? For example, how do they learn some material that is hard to learn from reading the literature and books?
Sorry, but there isn't an easy path through the research jungle. You just have to hack away at the obstacles as you find them. It is actually much easier now, with internet resources and electronic publication than it was, say 50 years ago. But inaccessibility of resources and difficult of interpretation has faced every researcher. 

Of course, some obstacles don't face every researcher. Doctoral students in the past normally needed a reading knowledge of at least three languages. That requirement made more papers available. My own dissertation advisor (mathematics) wrote, over the years, in five languages himself (Czech, Russian, Polish, French, and English). That language ability made some things easier. 

Every student needs the experience of not understanding a paper and working back through the references until the get a feel for what is being said. 

Your "blocks" may be different from my blocks, but neither of us is free of the need to just slog it out at times. 

But an even bigger need is that a researcher needs to develop insight into the very fundamental nature of a field. The resources aren't enough to guarantee that you will get this, and without it, you won't be able to extend what you find in the existing record. As you hack away at the obstacles in the jungle you also need a vision of your direction and where it might lead you if you keep hacking.