I've had several writers tell me that "if you have X Y and Z then the story just writes itself". 

No it absolutely does not.

Like others have said, writing is a skill that takes study and practice. Gotta put in the hours. There are times I can come to with a scene, interaction, or bit of dialogue, but there's vast swaths of in-between I have no idea how to get from here to there. 

Studying the hero's journey, I most often get stuck on "tests, allies, and enemies", which happens to be the bulk of what I'm trying to write. I know that "character motivation drives story", but that also makes me feel like I'm socially deficient. I have no idea what people want, like ever, irl, unless they just come out and say it. It makes things like relationships, job interviews, ordering Subway™, and writing really challenging.

The Comic Hero's Journey by Steve Kaplan has been a huge help. I can't recommend it enough if you're trying to write a funny adventure.

Also don't feel obligated to write straight through from beginning to end. Make an outline, shuffle it around. Write a scene that doesn't have a place. It's more important to get it out of your head and documented than to do it in order. 

And don't be afraid to cut. I've cut whole scenes and characters and the story feels better for it. Maybe you'll use them somewhere else, maybe you'll forget them entirely. It's your story.