If you've got a short linear story that works, but is too short, then maybe a subplot, something that breaks away from the main story, but interweaves back in might help.
A good example of this is the subplot about the man whose photos are torn up all over Paris, in the excellent film Amelie.
On the other hand, maybe this story just isn't feature material. Or, more likely, as I've found with my own writing, it is feature material, but only if I sacrifice the one part of the story I'm most attached too. Quite often the feature story is there waiting to be told, but it's buried under the short film story. I've just had to go through this with a story I've been working on for years. The scenes that I first wrote when it was as short, the ones that inspired me to write the story, have been cluttering up the storyline. Once I let go of them, the solid feature length story emerged.
I've found that in the transition from short to feature length, it is often the first act that suffers. Short films have to get straight into it and establish the story in a few minute, a feature requires a longer set up for the story and an opportunity to get to understand how the characters tick. If I dive straight into the conflict and then resolve it (short film), then I can't possibly get to feature length.