Is this a new screen? Sometimes it's difficult to get emulsion to stick to new mesh, even after you degrease it. Get a can of Barkeeper's Friend from the store (it's in the cleaners section next to Comet...) and use that and a green scrubby pad (a new one) to scrub the living daylights out of both sides of the mesh. Rinse it well until the water sheets off the surface. Finally, make sure you're exposing it long enough. Underexposure is much less forgiving on new mesh. Too much exposure is better than too little.