The middle is where most of a novel happens, which is why they're often so difficult to write.
For a long, long time in my early writing days, middles were the bane of my existence. I could start a story no problem, but once I got past the beginning, I narratively drove into the deep weeds. I don’t think there was a problem I didn’t run smack into when it came to middles–which is why I spent so much time figuring out how to make them work.
What makes middles so tough is that middles are where most of the plot happens. The protagonist tries and fails to resolve the story problem, the antagonist makes things harder and harder, the character arc unfolds, and all this stuff has to support whatever the beginning set up.
That takes a lot of work and finesse to get right.
Which is why the only thing harder than writing a middle is writing the ending. Except for writing the beginning.