I was a short story writer for many, many years, and it was extremely difficult learning how to write a novel. Though they're both fiction, the skillsets are quite different. A short story tends to have a simple story and only a few characters; a novel will be more complex, have subplots running through it, and may have a huge cast. My first book ran only 25K before fizzling out, though I thought I had enough material. It took a lot of work to finish a 90K book, and even then, I ran 60 pages short and had to add to bring it up. I expect the same thing to happen on my WIP, though I hope I've improved.
Anyway, this is what I learned on my travels:
A novel is not a long short story. I approached it that way, and I think it's one of the reasons the first book ran short. It didn't have enough of a story developed to sustain a 350-page book.
Individual chapters are not short stories. After treating it like a long short story didn't work, I tried this. Again, it wasn't getting out of the mindset of writing short.
Novels need subplots. Because I was writing it like a long short story or was thinking of each chapter as short stories, I wasn't thinking of things like subplots. I still have a lot of trouble with this one. If I think of them as subplots, the novel will crash and burn; if I let them come in naturally, it seems to work better for me.
Write longer. This sounds a little silly, but I tended to edit before it got on the paper, so I end up being spare (as described by other writers). It's not something I think I can overcome, so instead, I've been embracing it and trying to expand on using dialogue more.
Generally, what I'd suggest is making sure your story is big enough. Make sure you have a major surprise about third in, then one halfway in, and one about a third from the end. And don't forget a subplot or two.