Plots don't come the easy to me either +sigh+.
What usaully helps is acting out a beging to a plot you have made as you grow more deeply in love with the characters and story you will find an end for them. You can also just ask youself a ton of questions about you story and characters everything about everything has to have a meaning or reason, this gives you more detail about your story and streches your mind.
Elisabeth

Good plots flow out of well-developed characters, so it seems that you have a good start. In my belief, if your characters are not giving you plots, they are not well developed enough. Developing your world could also help a lot, too.

I usually think "Hmmm.... it might be interesting/cool/etc. if ____ happened." and go from there. I usually flesh out the plot as I go, so even though I'm the one writing, the story can still surprise me (especially while editing). My characters are developed as I write as well (although that usually results in out-of-characterness at the beginning before I edit).

Since you have characters already made up, maybe take a look at their background. They all have stories of their own, you just have to find them

I...don't think I've ever had a problem creating a plot. Usually, I start with a very small tidbit. (like the last free village being conquered) and then I create my characters, and just listen to them. They tell me what they would do in the situations.

And then I like throwing in mysterious little tidbits along the way. Usually, I have no idea how they will all tie together, but somehow they always do.

When I have an idea (which general come at the most inconvenient times) I just let it sit in my head for a while. A pot of water is a good analogy for how I do it.

You start by putting a pot of water on the burner (your mind). This water is the idea that came to you. It doesn't have to be much, but like water, it's the essential to the rest of the process. You leave it there, allowing the temperature to rise. (This is the natural change in the story that happens as you mind decides whether it does or does not like a certain concept.) Perhaps along the way you add some other basic ingredients, a main villain, a dramatic setting or too, but keep it basic for now. When it arrives at a boil (you'll know when, because the basic idea will be fully formed and acceptable to you) then you throw in the pasta (characters, places, etc.). At some point along the way you'll need to have made the sauce. (This is grammar, style, the basic rules of the writing world.) When your noodles have softened (fully developed in relation to the plot) then take them out and put them in the sauce. Now you've got your story.

I'd write the saying of French chefs here, but it would be too cliché.

It also helps if you can think about the plot a lot, in the back of your mind, and still do whatever needs done (i.e., school, chores, etc.).
I have found that even though I only started writing fantasy in November, I can come up with an idea, and work on it as I'm doing my schoolwork. Since I only come up with a basic outline (and then only in my head instead of typing/writing it out like some other people do) and figure out details when I get there, I often get stuck and have to think about what needs to happen next.

Maybe you can also look at your characters' personalities. Maybe you know that one of them is bitter, but is there a story behind why? Or why another character is afraid of a certain thing? (Which is kind of like I've already said, come to think of it....)

Usually my stories just pop into my head at random times (mostly when I don't need one! ), but that's not very helpful!
My most recent story kind of built itself off its characters. A while ago I decided to base a character off my Great Grandpa, since he was a really interesting person. Then I decided that he was traveling with his granddaughter, who happened to have a really fiery personality, and an elf who has been separated from his people his entire life!
So they basically came with a story, and I've been developing the idea while I do my schoolwork, like Sapphira said.

This is essentially what I did last night at about 11. I had a spurt of creativity and immediately started writing down ideas for the history, background and personality of one of my favorite characters, Cirwyn. If you've read any of our conversations in the character development section, you know that she is a very tough, anti-feminine and rough young woman.

I basically answered some of my questions about her: Why is she so tough? How did she meet Sangua? What is her reason for traveling with my other three characters? What are her weaknesses and struggles? What internal change does she need to undergo during the story?

Poor Cirwyn, she's now so emotionally damaged she'll most likely never speak to me again. It seems to me that emotionally hurt characters flow into great story-ideas. Mine is a bit, well, cliché but I hope to give it an unique twist.

Working on Cirwyn was a lot of fun! I'll post my "results" a bit later, but I want to let the water heat up a bit more, to borrow Neil's word-picture.