Literature can be challenging to research because there simply is not as much written about it as many other subjects. Here are some more things that can be challenging when researching literature:
There are Generally Fewer Articles on Contemporary Authors
It is easier to find criticism on authors from the past than on contemporary authors. They have not been around as long to study or have had sufficient if any critical articles written about them. There can be exceptions to this.
There are Generally Fewer Articles on Women and Minority Authors
Authors who are women or minorities from any era have had less written about their works, though that is changing. There just may be less available
Some Works Just Don't Have Much
There are some works about which there is not much written, even some works by the most famous authors.