Sometimes, a story doesn't come together with the right combination of characters, plot, and theme. That doesn't mean the story won't ever be workable. But getting stuck because this story isn't "the one" isn't unusual--and it's not shameful.
1. A story's map can be a powerful tool in thinking about a story. You may have gotten off a main trail without realizing it, and you're now stuck in the woods. If so, backtracking and telling the story a different way may get you back to the events of the plot. (When hiking in unfamiliar territory, map readers are taught NOT to hike towards the end destination. Instead, they are to hike to the nearest point on the map they can see from their vantage point. Once there, they take inventory and pick another point closer to the destination. This is a useful concept when figuring out a plot for the first time.) If you're still stalled, consider that you have chosen a hiking trail that is too easy--you need some more excitement. (This is usually where you crash a wrecking ball into the hero's apartment, or throw Huck and Jim off the raft and into the mighty Mississippi.)
2. The Battle Of The First Twenty Minutes. If you can make yourself sit and write, no distractions, no excuses, getting through that initial block can open up your creativity. Even if you don't think the writing is perfect, keep writing until you get past the first half hour or so.
3. Write what you're passionate about. Write the characters that won't let you sleep at night. If that's not the current story you think you should be working on, save that one and write the one that's bugging you. You will find that your writing is that much more meaningful when it matters to you. :)