Breakthroughs are not predictable, but we can increase the odds in a few ways.
First, step away from the problem, literally. Get outta the chair, head outside and keep walking. Exercise and fresh air, not to mention a change of scenery and cognitive focus, are good for the brain.
Also try solving other problems and puzzles. Smaller things. Get back inside your comfort zone and build up wins; the momentum to charge once again at the big challenge at the edge of your skills.
Sure enough, NDK is becoming clearer to me as I meet new challenges head on.