Now I am going to sound like a broken record, but it was TLJ. TFA progressed the characters so that by the end Finn and Rey had decided to stop hiding/running and embrace their role as part of the direct fight against the First Order. The characters are in a perfectly logical place for the protagonists at the end of the first move in a trilogy.
But then TLJ doesn't grow them from there. It regresses them, or maybe takes them sideways Johnson has Rey trying to bring Luke back so that she doesn't have to be the Force user... but also, she's trying to get him to train her to use the Force... and then there's this weird, icky Reylo element. So her character motivation, her challenges, and her goals all became really muddy.
The storyline was almost there--there should have been tension out of Rey NEEDING the reluctant, bitter Luke to train her so that she's able to fight a full-strength Kylo. But IMHO, Johnson never gave us the feeling that Rey actually needed or even cared about Luke teaching her. Heck, he doesn't even bother to tell us why she stole the Jedi manuals. Was it because she'd given up on Luke's help and planned to use them to teach herself? Was it because she felt they belonged with her as the next Jedi? Was it because she thought Leia could get something useful out of them? Was it guidance from the Force calling her to do it in a manner similar to when she went to the cave or to Anakin's lightsaber?
Finn's storyline in TLJ is even worse. Johnson didn't seem to have any idea what to do with him or who he is. Finn doesn't question Poe at all about Holdo, even though what Poe is telling him and Rose to do is a huge thing. Finn gets lectured about the evils of the First Order from Rose, as though we weren't introduced to Finn with him breaking his brainwashing after having to partake in a massacring a village! Finn's big moment is defeating Phasma and declaring himself to be Rebel Scum... but he already defeated Phasma at the end of TFA and she's been off screen the whole time and Finn has had no direct conflict with the Stormtroopers and Finn is actually partaking in a mutiny against Resistance leadership (that is going to result in the devastation of the remaining Resistance) and Phasma's defeat is going to be absolutely meaningless for the First Order vs. Resistance storyline anyway.
TFA left Finn in an interesting place as an ex-Stormtrooper who would be looking to find his place with the enemy side. We should have seen him struggle with the differing culture, political system, assumed beliefs. That, in turn, would have let him serve as a POV character to flesh out that political worldbuilding, which was the weakest part of TFA. Instead, we got Canto Bight.