The thing about calling The Lord of the Rings a trilogy is that it isn’t. It’s either six books or one, depending on how you look at it. Sure, it was published as a trilogy, but each book of the “trilogy” contains two volumes of the larger story. And each volume is a complete narrative arc with a rise and a climax and a resolution. I think that is part of why it is sometimes wobbles when adapted to other contexts. (It’s certainly why the Peter Jackson movies got a bit jumbled at times, especially in The Two Towers.) It can be a bit difficult to make a good adaptation of something that has two equally momentous climax points.
In Fellowship, people mostly deal with this by cutting out most of the first half, (goodbye Tom Bombadil and the Barrow Downs) and making the confrontation with the Nazgûl at the Ford of Bruinen part of the rising action instead of the climax. This is relatively manageable although I think it sells the story a bit short since the Hobbits are very much the main characters of the whole thing, even when it steps aside to focus on other characters.
In Return of the King, by nature of it being The End, a lot of threads are coming together and it is a bit easier to weave them together into something that makes sense.
The Two Towers though. That’s just two fully separate stories. The story of what’s happening to Merry and Pippin in Rohan and Fangorn and Isengard, and the story of what’s happening to Sam and Frodo on their journey to Mordor. Furthermore these are two stories that are tonally pretty different whilst still being part of the same overall story. The Merry and Pippin story has a political/military focus, and of course the battle of Helm’s Deep and the destruction of Isengard. The Frodo and Sam and Gollum story is more or less a road trip relationship drama. That’s funny, but the point is that its focus is very much on those three characters, their relationships, and the strain they’re under. It’s very personal in a way that the Merry and Pippin story isn’t, simply because there are so many more events happening in the Merry and Pippin story. It is difficult to weave these two together in a satisfying way! In fact, Tolkien doesn’t! He tells the Merry and Pippin story right through to the end, and then switches over to Sam and Frodo’s and tells that straight through.
Anyway that’s why a lot of adaptations of The Lord of the Rings falter a bit in the middle. And also why you should read the books again. They’re great.