Have you got an oven thermometer? You could check to see if your oven gets to the correct temperature, they're not infallible. Don't overbeat egg whites and treat gently when folding in! Don't open the oven door before time.
I've only made one once, years ago, and it was fine, but it was a large one.
Thank you for the tips on here. I don't think I'll be auditioning for Masterchef anytime soon! The soufflés didn't really rise properly, and inside they were like scrambled egg! 🙈 Luckily I had plenty of ice cream in the freezer instead. Back to the drawing board.