Why does the film positive sometimes stick to the bottom of my screen causing the ink to come off the film and stick to the screen?

The problem is not necessarily with the ink jet positive. It may be with the drying conditions where the screen are being dried and stored. Ink jet films are extremely sensitive to any moisture. If the emulsion is not completely dry the film will wick out the moisture from the emulsion, especially in a vacuum frame, causing the ink jet positive to attach to the screen. Dry the screens better and store them in a dehumidified area.