If you have n facts in your mind, that means you have n(n-1) pairwise combination of facts. That is a lot. Add in more facts necessary to generate an idea, and the number of combinations grows, well, combinatorially.
Once two facts have been raised to your attention, you can judge their combination trivially. The challenge is finding the right two facts, and that can easily be seen by noticing your search space.
How do you get better at searching through your search space for creative ideas? Well, ask creatives- that's what they do all the time. I've found A Whack on the Side of the Head mildly useful on that front, but don't know how it compares to other books.
Today I learned that you can toast marshmallows in the oven.
By "learned", I mean "I read a recipe which included as a step toasting marshmallows in the oven". I didn't have to try it out to realize that this would obviously work. It was plain as soon as I heard the idea. And it shouldn't have needed pointing out. I know how ovens work. I am familiar with the marshmallow species of food. I love roasted marshmallows while hating them in most other forms and often occurrently lament the difficulty of arranging open flames over which one may safely toast them. I routinely try new things in the kitchen to get results I want.
And yet I read it, and was surprised. And so were the people I reported this finding to. It needed pointing out.
What other facts need pointing out, although they are plain on inspection? What is the pattern behind these facts and a good way to find more?