I don't have too much of a problem creating short stories to order, in fact often some kind of random prompt gets my brain working. The short I sold to Andromeda Spaceways Inflight magazine started life as a fun activity on a Babylon 5 Usenet group. The challenge was to write a story that would include all the titles of the episodes of one season. Thus the story was created around a random selection of words and phrases. Most of them got edited out when I decided that the story was worth polishing and submitting, but there may still be one or two lurking in there. :)
I'm also not having too much difficulty thinking up stories for the exercises and assingments for the creative writing course -- well, apart from the terminally dull suggestions that assume we're writing a mainstream or literary novel, but even there I managed to put a boring accountant on a moon base. In fact subverting what's expected is a profitable source of ideas. The one I'm working on now for the second assessed assignment is based on the prompt "an invisible wound". The hint given said, "‘an invisible wound’ is likely to revolve around one or perhaps two people affected by a secret." Oh, no it's not, not in my case. Letting my brain lose with the phrase "invisible wound" sent it off in entirely different directions. :)
Another useful method of generating short story ideas is the random plot generator, which I found either via a mention on Usenet or here, I can't remember which, and a random word generator suggested by the tutor on the OU course. I used both to give me character and scenario for another writing exercise about a gold miner.
So it's not the idea part I find difficult for short stories, it's actually getting the thing written, especially in finding a good ending, one that isn't trite or too obvious or one of those endings where the story just stops and the reader goes, "Huh?"
I also find them much more hard work, per word, than a novel. They never manage to build up any momentum because by the time they've got going, they're over. I seem to have to push them all the way, uphill, both ways. In fact I'm poking one now that I need to finish by Christmas because it's for one of the assignments for the creative writing course I'm taking, but it's being very coy and only giving me a few words per day. So far I've already done as much research -- including a trip to visit a castle and take photos -- than for any novel and I still only have 254 words to show for it. Just another 2000 to go!