One idea isn’t enough.
If you’re writing fiction, you need two or more concepts in order to have a viable premise. But they can’t be just any old concepts. Preferably, they should be ideas in opposition to one another or which interact in such a way to generate plot in an "automatic" fashion. Many’s the time I’ve thought I was ready to go on a project, and then I ran out of steam. This is because I didn’t have two or more ideas interacting with one another to create literary fission.
Often, the underlying concepts in a successful story are not only dissimilar, they’re from two very disparate places in my life. For instance, I might be spinning my wheels on an epic space opera when I come across a nonfiction article on weasel infestations. It will then occur to me that what my pulp masterpiece needs is a new threat to galactic tranquility. This will come in the form of — you guessed it — angry star rodents.
As you can see from the above example, the concepts synergizing their way through a story can fall into two categories — broad and specific. The notion of doing an old fashioned space opera is broad. It is a choice of genre as opposed to an idea based on particular happenings from within the tale. The introduction of weasel-ish foes is more specific. Hopefully, it will draw upon elements of the nonfiction piece which sparked me in the first place. If that article has a breakdown of weasel society and why it is they have a predilection for infestation, then perhaps I can graft those details onto my rodential antagonists. This may serve to enrich what otherwise might have been a run-of-the-mill pulp actioner.
Of course, my sci-fi example is silly, and the idea of it taking two ideas to propel a story is not an absolute — it’s just very nearly an absolute. Is it possible to write a story with only one driving concept? Sure, but I believe such tales to be the exception rather than the rule. If you’re having difficulty writing a story, step back and ask yourself, would I benefit from having two or more ideas grinding against one another providing grist for the mill? You most likely would, but sadly, finding ideas that cohere is not an exact science. Generally, this combination of notions results from an intuitive leap — an ah-ha moment. Forcing the issue and jamming two ideas together which do not belong will probably cause more harm than good. Just sit back, relax, and make yourself an antenna.