I have always thought you can't set out to win a short story contest. You must just write the best story you can write, everything else is out of your hands. This article says what I've always thought.
I’ve entered, judged and managed short story prizes before. Each one is a crapshoot. You’re lucky if you win at all. You know why? And here’s the reality – the winning short stories are the ones that every single judge likes. Do you know how hard it is to get one person to like your stuff, let alone four or five judges? If one judge is really passionate and the others are so-so, your story won’t win. If everyone wants it to win except for one person, it won’t win. Usually, it’s everyone’s second or third favourite story that wins – that’s because everyone has different tastes, which is subjective anyway.
I use short story contests as motivation to write a new story. If it does okay in the contest, that's just an added bonus. Just write folks, write the best thing you can. Everything else will sort itself out.