That's the first clue. We think we wouldn't want to write a whole novel about someone so extreme, and therefore our tendency is to make our central character, the one we want all the readers to like, too bland. They become over anxious to please, pawing at the writer with big love-me eyes. Irritating in real life, irritating to write. No wonder we start getting caught up in the character who doesn't give a damn what the reader thinks of them. So the first thing to check for is: is your main character too bland?
The second thing to look out for is active or reactive. Main characters drive the plot, not react to it. If your minor characters are providing the plot lines and your main characters are reacting to it, no wonder your minor characters are taking over. They ARE the main characters. So take a good hard look at your novel - who is driving it forward? If the answer is anything other than the characters you think are your main ones, then you're going to either have to change them to make them more active, or shift the story focus to your minor characters.
The third and last thing is related to the first two. Is your main character's problem interesting enough? Does it really matter to them, and to you? We are all the stars of our own lives and our problems are vitally important to us, but they're secondary to others. (Which is why it's so easy to dish out advice to other people - 'leave him!' or 'tell her what you really think'.) Your main character's problems have to be as real and important to you as your own problems are to yourself. Because if they're not, your reaction as a writer will be the same as listening to a friend talking about some problem they've got which doesn't affect you at all. Your attention will wander off to minor characters who seem more, well, fun.
So there we are. Three things to think about. I never said I'd be providing a solution to this one, just stuff to think about.
