There are two different reasons why people become writers. Some people become writers because they like the idea of being a writer. They like to play with words, they love the language. These writers usually write beautiful, poetic prose, but the story is usually not their strong point (I'm generalizing here, don't bite my head off). To them, the words are more important than the story. Other people become writers because they have something to say. They probably didn't consciously want to become writers, but they had so many stories inside their heads that the only way to quiet them was to write them down. These writers usually don't have a flowery writing style, but probably a more simple and straightforward one, depending on how much they care to work on it, but their stories are inspired. They don't open a word processor and go "ok, I want to write something, what should I write about", the story is already there in their heads, just waiting to be written. The point of all this is, do you have a novel length story to tell, or do you just want to write something, anything?
Have you heard of National Novel Writing Month? It's an event that happens in November where people from all over the world attempt to write a novel of at least 50000 words in 30 days. You might want to give it a try next November (or even next month, they're having a NaNoWriMo Camp). The point of it is not to produce a good novel, but to produce a first draft. The point is to prove to yourself that it can be done, and also to teach you a way of writing where you kill your inner editor and just write, because you simply don't have the time to edit, or even to think too much about it, you just write. When you start, you already know it's not going to be good, not at that first draft anyway. It might actually turn out to be total crap, it doesn't even matter. You might think what's the point then, but it actually takes the edge off. It lowers your expectations and you end up writing just for the hell of it. Plus it's fun hanging out with other "WriMoers" on the official forum and other places while you write, and if you get stuck, there's always someone to help you out. And then if you make it, if you write those 50000 words in a month, and you find that they have potential, then the real work starts. There's a saying that great books aren't written, they're rewritten, and it's true. You need to edit a novel three, four, even ten times to make it good enough for general consumption. There are quite a few best-sellers that were written during NaNoWriMo, like Water for Elephants and The Night Circus.
But to return to the first point of this post. I've done NaNoWriMo for the past four years. First two times I've used a story that came from a dream, that was with me for several years already but I've never bothered writing it before then. NaNoWriMo was a perfect excuse to write it, and it was hard, the story was only vaguely fleshed out in my head before I started writing it and I had to come up with a lot of it on the fly, but I did it. Next year I wrote the sequel, also without much preparations. Third year, though, I wanted to try something else. I came up with a really fun story, I invented awesome characters with character backgrounds and everything, I outlined in details almost half the novel before NaNoWriMo started, but when I started writing, it fell apart half way. Not because it was bad, it was maybe even better than the previous story, but it felt fake, phony. It wasn't a story that came to me, it wasn't inspired, it was something I tried to make up, to force. I suspect you have the same problem. Ideas for short stories come to you, they're inspired, they write them selves freely, as you said. But from your question, I have a feeling you're trying to force longer stories, and subconsciously you just don't see a point of telling such a story, that's why you get writer's block. You can plan it and plot it all you want, but if the story doesn't feel true to you, there's no motivation, no desire to write it, characters don't feel real... I'm not saying you can't write a good "made up" story (yeah, I know, all stores are basically made up, but some are more made up than others, if you know what I mean ;) ). The first type of writers, the ones that write for the sake of writing and not because they have something to say, they're usually good in "making up" stories. But with some stores, when you read them, you can just feel they were inspired, that writer was compelled to write them. They have that something special you can't quite explain.
So, if I'm right and this is the source of your problem, you need to let the story come to you, and not try to force it out. Maybe you could, like jersey suggested, make a collection of short stories with a same theme, or same characters, or same setting. Maybe you could take one of your short stories, something with characters you find really interesting, and try to build on it, to expand it. Some short stories just scream to be allowed to grow. But don't try to write it until the idea grabs you completely. Give it a month, half a year, whatever, to mature inside your head. If after that time you not only still want to write it, but it has also fleshed out in your mind, that's the story you can write, that's the story that will write itself.
Or maybe, in the end, you're just not made for longer stories. Just yesterday another writer complained to me that she can't write short stories because all her ideas for short stories have a tendency of growing into giant monster ideas. We're not all made for everything. Some of us are better at short format, some of us at long. You're no less of a writer for writing short stories. Some of the world's best writers wrote their best stuff in short story format.