If your retainer doesn't fit anymore it's usually because your teeth shifted after removing the retainer.
But if not maybe your teeth and mouth have just grown especially if you're still a kid or teen and are still growing and if the retainer has stopped fitting your teeth and mouth correctly then you should speak with your dentist about it.
You'll need to have a new retainer fit otherwise the retainer that doesn't fit will do more harm to your teeth than good and mess up your teeth.
So speak with your dentist and let them know that the retainer you have no longer fits and they will get you another new retainer fitted that does fit and works correctly.