For me, it's simple. Short stories. I can't write them. For the life of me, I can't do it. I sit down, ready to write and one of two things happen. The story dies and quick and ugly death before the first sentence is done or it bloats so bad it resembles a Stephen King novel. My first story was supposed to be a short story account of my time at Job Corps as a teenager. It finished at just under 194,000 words. So, yeah, the short story is my bane. And I have no clue how to solve it. But hey, writing is fun no matter what so I'm not in a hurry to figure it out!
Something I find that helps with short stories is just doing very brief scenes. Those are the only ones I can complete anyways and they could definitely be fluffed up to full blown stories; but I can at least keep them to less than 10k