I don't mean to be picky about fanfiction feedback, but if your response is "I don't understand a single thing going on here", maybe don't?
On one hand, I understand and accept responsibility for some amount of confusion. I like to write long stories. The story in question is a longfic sequel to longfic with many OCs, idiosyncratic terms, headcanons, and sometimes hiatus between chapters. Right away, cosmic horror workplace drama with psych-thriller elements is not going to be the most accessible thing in the world. And I'm autistic! I have to be realistic and understand that most of my work asks more of readers than a one-shot where only canon characters are named, written in extremely straightforward language.
But going back to review something you missed, or pointing out specific things that confused you, are also both options for a reader. If you opened a volume of any long, stylistic book series to a random middle chapter and then proclaimed, "I don't understand anything that's going on," who would expect otherwise?
Fanfiction is free, and I'm not paid to do recaps of what information to remember before any given chapter. It's not a TV show and I don't benefit from anyone binging it or not. I'm not going to create a glossary or a wiki for my work. If someone won't review prior chapters, they won't read those either.
I will never be so arrogant to say that improvement isn't a priority, that communication effort is all on the reader's hands. I'm way too autistic to think that works out for me. But if you're bold enough to actually tell me, "I understood nothing" then perhaps you're bold enough to lock in?

















