From what I've learned about brains, the left brain is engaged in symbolic thinking about a problem, which engages more logical, methodical problem-solving. For a combination that you won't arrive at through that approach, you have to give your brain, apparently involving activation of the right dorsolateral prefrontal cortex and other right-brain parts, more time to integrate info from stored memory or lower-level processed stimuli or to make novel associations related to the problem. When left prefrontal cortex is engaged in focusing on performing a task, it'll inhibit the processing of info seemingly irrelevant to the task. This is why aha/eureka moments are more likely when you're relaxed, not focused and your mind gets to wander (e.g. getting on bus while on vacation, taking a shower/bath). Studies suggest that more creative or sudden-insight (as opposed to deliberately trying different combinations) problem-solvers have greater right brain activity and lower inhibition of it.

Look up "Aha! moments" in the index of Eric Kandel's book, The Age of Insight, which cites many papers, incl. "Explaining and inducing savant skills: privileged access to lower level, less-processed information". A few of my other references: "Bayes for Schizophrenics: Reasoning in Delusional Disorders", "Creativity tied to mental illness", "Through the Wormhole: Creativity Cap"