Check your rollers. At the bottom of the Polaroid film is the big white rectangle, this is where the chemicals are stored. If your rollers are dirty or glazed then they won't spread the emulsifier correctly and your prints won't come out. Open your camera with no film in it, and clean the rollers with rubbing alcohol, the stronger the better. Use cotton swabs. Chemicals get all over them and gum them up. Clean them and check that they are rolling well. Make sure your film cartridge is well seated and then try again.
Don't shake the film it doesn't do anything.