This is an interesting thing that I've started to notice now that I have a year of publishing under my belt (yeah, I know, I'm a total veteran). Yes, I have noticed a gradual increase in sales as time goes by. But I've also noticed that the stories that have been out the longest are my best selling works.
I almost never sell new stories. When the story is first put up I'll get maybe one random sale probably from being ont he new release list. But other than that, nada. At first this really disheartened me. I put all this work into my short story, so why the heck isn't it flying off the shelves??
Well, apparently short stories have to age a bit. Ferment. Pickle. Marinade. Clearly I have food on my mind. I now am at the point where I sell things consistently, but the things that sell are the very first stories that I published. I have slowly noticed an increase in other stories but, again, these are the stories that I published right after my first few.
This is something to maybe keep in mind. If you write short stories you already have to embrace that they are not going to sell like a novel. I've talked about this on numerous occasions on this blog. But I think it's important to keep track of which stories are selling.
Don't dismiss a short story just because you published it four months ago and you only sold two copies of it. There's a good chance the story may still sell. It just has to age a little longer before it finds its audience.