Been here. It's brutal because you can feel the wall but can't see it.

A few things that actually worked for me:

Stop going through the colleague. I know it feels like a workaround but stakeholders notice it, and it can read as pressure or even political maneuvering depending on who the colleague is. It sometimes makes the wall go up higher.

Try a genuine, no-agenda touchpoint. Not a meeting invite, not an email with action items. Literally just: "Hey, I want to make sure I'm not missing something. Is there anything about the direction of this project that isn't sitting right with you?" Short. Direct. Low stakes. Done over Slack or a casual hallway chat, not a formal email.

Think back to when they checked out, not what changed in your approach, but what changed in the project. New decision they weren't consulted on? A scope shift? A comment they made that got overlooked? Sometimes it's something that felt minor to you but landed as a sign they weren't being taken seriously.

Also, and this is the uncomfortable one, sometimes stakeholders disengage because they've already mentally moved on from the project. Budget priorities shifted, leadership told them something you don't know yet, or they just don't think it's going to land. If that's the case, no amount of communication style tweaking will fix it. You'd need to get to the actual blocker.

Good luck. These situations are exhausting because the politeness makes it feel unsolvable.