However, I have since found the secret. It's world-building. Take some time to elaborate on the story and CREATE the story in order to get out a good-sized wordcount. Filler scenes aren't any good. They add nothing, and unless you're a wizard then they only put the reader to sleep. So, then, what is world-building?
As an author, you may not know your characters or setting that well. That's fine. In fact, that's the point. Write the scene, but do not deviate from the story. It's tempting to avoid what I'll call "the darkness" of the story. It's unknown territory, even to the author! But since you ARE the author, you're the only one who can shine the light in the darkness and expose your story's world.
So confront the unknown. You don't know how that character should act? You don't know their job, their personality, their birthday, their home life, their hobbies...? Each one of those is a branch that offers even more branches. Like taking your light and burning those branches with it to light up the darkness of your story. Oh, so your character works here? Is it a popular place? Who else works there? Do they get along? Answer all of these tangential questions and more and more will pop up. Don't hesitate to ask yourself; don't stop thinking!
Before you know it, you'll have answered all of these, and when addressing them in the story you'll realize that you've suddenly gained a gigantic amount of words added to your wordcount. Naturally, there is also an element of balancing out how you convey the information (infodumping isn't good) but as they say it's better to have tried and failed than to have never tried at all.