Question: I'm 1/4 through my first novel, with a pretty solid 12 page outline of the whole thing. After years of reading books about story structure, I think I have a good grasp on how to make a plot work.

My characters, however, are turning out to be quite flat, uninteresting and generic. They do have motivations and I'm putting a lot of effort into making sure they act accordingly, in a logical way according to their goals and the story seen from their perspective, etc. But they don't feel "alive".

I've read about the archetypes in The Writer's Journey, but that's about it. How can I make my characters come to life?

Answer: 
Finish the book, then go back and look at the characters again.

When I start writing my characters are fairly generic, flat, and frankly cardboard. Character is my weakness, I love plots and ideas.
The crucial thing is that in the second draft, I then go back and add in bits of the colour, life and uniqueness which the characters have only accumulated by the last chapter.
Sometimes this will require changes to the plot (I'll realise that Rachel shouldn't have done that in Chapter 2, it doesn't fit with a part of her character I only realised in Chapter 7), but overall I find this a much more organic and natural process than trying to specify everything about a character before I know them, and as such the results tend to be much more convincing and compelling.

Your mileage may vary, but I think if you're a quarter of the way through without your characters having sparked yet, you're probably more similar to me in this respect than most!
Your problem may stem from the fact that you are writing a book too much "by the book." Characters aren't like bits of code. Unless they walk around in your subconscious, speaking and breathing and acting like real people, they won't seem like real people on the page. If your characters don't surprise you, they won't surprise anyone else. Novels are about people, not structures, and as a novelist your job is to find a structure that fits the people in your story, not the other way around.