So when you were doing that, what would make you go "nope, next?"
For specific things: awkward writing (which doesn't happen too often). I'm really tired of serial killers and zombies so if a story has those then it needs to make up for it in some other way, it could be an interesting new angle or perspective, or particularly clever language, or really anything to show that it's not a member of the rank-and-file.
But most early-nopes boil down to the general category "didn't catch my interest". I usually give a story at least a few hundred words to grab my attention, which is more than some readers will do. If I can set that story down and not wonder what happens next, then it's probably not for me--I've experimented with reading further after I get that initial sense, but I don't recall a case where my initial disinterest was reversed.
The cause of that can be a variety of things, and it's important to realize that such disinterest is largely a matter of taste--I have similar disinterest in a fair amount of professionally published stories.
My interest can be triggered by a variety of things--a character I can empathize with, a weird and fun situational idea that I haven't seen before, a cool speculative element, clever or funny use of language.