I find it really easy to come up with characters that (I think) are interesting then have the devil of a job working out "what to do with them" - i.e. what sort of plot's going to happen to them to test their mettle. Motivations, personalities, strengths, weaknesses, quirks and "foil" characters who're equally well thought out - all come very easily to me.
Then I get stuck with, "OK, I've got a couple of interesting people who'd interact in very interesting and fun ways, so what is the story going to be about?"
I'm certain no one wants to read about how these two spend their days working in the same office trading witty repartee; that there's got to be some sort of plot that throws them into the fire and forces them to work together - often, I have a hell of a job finding that plot.
I've currently got more characters than I have workable plot ideas for.