It probably has a lot to do with the plot of the short. Since I have no idea what it's about, it's hard to specifically help, but here's a few suggestions.

Add a subplot. It can mirror the main action, whether it be the main character's love interest, or his art project, or his work.

Write more plots and create an ensemble film.

Fill out the plot. Hitchcock's Rear Window could have been squashed down to a twenty-minute short, but instead he pulled it out over a few days. Obviously, it's going to slow down the film, there's no getting around that, but there's going to be more of it.

Let's say I wrote a short where drug dealer's friend overdoses on his own pills while staying at her house. She freaks out and has her friend help her bury him. I could easily flesh this out into a full-length film by adding a drug network plot, so that the main character is trying to keep her place in the network by hiding his body. Before he dies, she could rise in the drug world, and his death would be a major problem, so she buries his body. Then after that someone finds it and she has to go on the run.

Now, whether or not I want to flesh it out is another matter, but that's just an example.

Think about all the little details in your short and how they could be expanded upon.

I left my heart in Pruitt-Igoe

Yeah, I am working on that as we speak. I think what I will have to do is dismantle the whole plot, scene by scene, and then rebuild it in a different fashion. A bit like if I were to add more bricks to the centre of a wall, I would have to knock it down first then use those blocks and more to recreate it. It is a shame though, because I did love the plot of my short, even though it was very simple and with scarce twists to the tale, but at least this way I can keep most of the dialogue...and I'm dreaming up some more right now.

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You can't keep 'em out, they're already in!