I agree with what @Karababy52 said but I'd like to add what I read a long time ago when I also had the same issue XD
Apparently if you have the sim play the game you want to enter a tournament in before, like let them play the game normally for a couple of hours and then enter the competition. I usually always got first after I did that (as well a being focused etc) but I don't know if that was the reason. I tried to actually let them play the game until the action was over, without me cancelling it out before it has run its time and then enter the competition, but it takes a long time.
I also read that the more your sim plays the game, like overall, the more likely he/ she is to win. Like if you enter a competition in a game your sim hasn't played in a while you're more likely to end up second or third etc. I don't know if there is any actual truth to this as I haven't checked the game code or anything, it's just what I read :)
