The cause of the issue depends on what you mean by “not developing”. If the photo turns out all white then that means your film has been flashed - or exposed to so much light it’s no longer able to record a photo. Film works by being exposed to an image projected onto it through the camera’s lens while the shutter is open for a fraction of a second. Therefore the film must otherwise remain in total darkness until it gets pushed out of the camera, during which the packet that contains the development chemistry gets broken open and spread across the film by being squeezed through a set of rollers. The black card that sits in the front of the film pack and gets ejected the first time you push the shutter release is commonly referred to as a dark slide - it’s there to protect the film from light so you can load it without damaging it by exposure to light.
If the photo turns out all black, or some photos turn out all black and others turn out just fine, then your Instax camera has a defective shutter. For some reason, it jams just slightly and fails to leave an opening. The film ends up not getting exposed to the image and that’s why it turns out all black. This seems to be a recurring issue with the Instax Mini 9, and maybe other models too. The camera has a one year warranty so you can get a free replacement or repair from Fujifilm. Also contact them to get any film that was wasted replaced. Or return the camera to the store you bought it from for a refund if that’s their policy and buy yourself a replacement. That’s how I solved the issue with mine.