There's a peculiar kind of reading experience where you finish a chapter and realise you can't quite remember what the main character did in it. The plot moved. Things happened. But your protagonist somehow wasn't in it. They were just present, the way furniture is present.
This is one of the more difficult problems to diagnose in a long story because it doesn't announce itself. Nobody writes a scene thinking, 'I'll make my protagonist irrelevant here.' It creeps in, usually under pressure from the story itself.
Readers build a working model of your protagonist in the first few chapters. They accumulate a sense of how this person moves through the world. What they reach for when things go wrong. What they avoid. What they'll compromise on and what they won't.
That model is what lets a reader inhabit a character rather than just observe them. And it depends on consistency of decision-making logic.
This isn't the same as consistency of behaviour. Your protagonist can act out of character — people do. They can surprise themselves. But there needs to be a through-line of why that the reader can follow. When a protagonist makes choices that feel arbitrary, or that exist primarily because the plot requires them to be in a particular place at a particular time, the reader's working model starts to break down. The character stops feeling like a person with an interior life and simply becomes a vehicle.
The fix isn't to make your protagonist more consistent. It's to know their decision-making logic well enough that every choice — including the surprising ones — is legible in retrospect.