I did a website for a friend, a kind of a knowledge base and installed google custom search on it. To my surprise google stubornly kept showing no search results. I redid the custom search settings, paying close attention to the domains on which I wanted the search limited on, but still no results.
I took a quick walk on the Google Help boulevard but I didn’t find anything. A google search later and I found numerous causes: wrong definitions for the domains on which the search is limited, wrong integration of the code, wrong definition of the result page the form should point to.
My problem was another one: google finds no results on a page it has not yet indexed! I know it’s true because over time, without modifying anything on the code or google custom search settings results started appearing.
So you can add another cause on the problem list.
Have fun googling!