[Here's a quick replay of a WRITERLY post from a few years ago. Hope it helps.]
I have some things to say, quickly, to those who are writing stories and want to make the move to the novel. Actually, I've got lots to say about this -- bolts and bolts of fabric lining the walls -- but I'm going to give the small package version.
First of all, it may well be true that you're tromping the terrain of a novel without knowing it. Try to look at your characters specifically. Is every woman character of a certain age really the same woman -- your mother? -- that you keep dressing up in different pantsuits?
This is possible. Look for your recurring characters. See if they fit together. This could be a very good discovery.
Now look through the stories you've written for the ones that hold the most heat -- the ones with the meatiest characters, with the strongest undercurrent of longing, suffering, need.
Then think in terms of the story as container. What keeps a short story short? Things like: size of cast of character, passage of time, point of view (the dissolution of a family from one point of view can be a story, same dissolution from 3 points of view is a novel), geography, heft of insight, limited retrospect, limited backstory ...
Is there a place in your story that can be opened up -- I call this looking for pleats. In my first novel -- originally an 11-page story -- the narrator's father has an affair with a redheaded bankteller, is confronted (in a weird way), and disappears for the night. In the novel, the most important pleat was that one night. Why not have the father disappear for a summer? What then?
I also filled in a present day for the narrator so that she had a reason to be looking back at the summer of her father's disappearance. I created a novella-length present day plot for her, expanding retrospect. In the father's absence, the mother and daughter road-trip back to the mother's childhood home ... expanding geography and cast of characters. I was then given a reason to write the mother's backstory. I kept the singular point of view.
Of course, this is just one way of getting at it -- a pinhole view.
The unforgiving truth is that each novel teaches you how to write it.