It is always possible that the temperature is not ideal for the picture to develop. If you are using the film originally designed for the camera then the film is really old and not likely to work. If you are using the Impossible film then the film is very experimental and very sensitive to its surroundings. You don't want it to be too hot or too cold. In the directions of the film is says that at an ideal temperature of 70 degrees Fahrenheit it will take the pictures about 10 minutes to develop.

Here are a couple other resources for troubleshooting similar cameras that might help:

Polaroid Onestep Close Up Troubleshooting

Instax Mini film not developing