I have been using some expired Polaroid 600 film from 2004 and all of my images come out with uneven development. I know the colors are bad and there's nothing I can do about that, but what I want is just to increase the chemical spread so that I can use more of the film frame. Is there any way to do this?
The reagent that starts the development process is stored in the wide border at the bottom of the print and spread onto the exposed area by a pair of rollers in the camera as the print is pushed out. There are a couple of possibilities for what's happening in your case:
One is that the rollers in your camera are dirty and aren't doing their job. You should be able to see them on a 600 camera when you pop the front open to insert the film pack. If they're crusty, a paper towel dampened with warm water should get rid of it.
The other is that the film is approaching the end of its life, and what's in there either won't spread or is spreading and is no longer effective. There's not much to be done about that other than finding other film. If you're not already aware, The Impossible Project is producing Polaroid 600 film.