Where are you letting your coated screens dry?
If the coated screens stay warm for a long time they won't expose correctly.
If there is any light in the room where the coated screens are drying, they won't expose correctly. You can have the lights on while your coating the screens, but they need to be in the dark while they are drying. Even a tiny bit of light for the 12 hours you are letting them dry is still 12 hours of light on the screen.
If your art is not a real good solid black, you are going to have a hard time getting your exposure correct. Too little exposure and your art won't hold in the screen, too much exposure and a weak original will over expose and you won't be able to wash out. Try printing two transparencies and taping them together to get a darker original.
The Speedball is very 'durable' meaning you don't have to be hyper-vigilant about casual light exposure to your emulsion or coated screens. Keep the emulsion in a cool dark place for storage. Keep your coated screens in the dark while they are drying. Don't take anything outside in direct sunlight. Minimize all other sources of light to the emulsion as much as possible. If you can, get a yellow bug-light from your local hardware and use that in whatever room you are working in for your dark room.
And to give you at least a bit of a target.... for speedball emulsion I use a 250 watt photoflood in a reflector at about 15 inches for about 8 minutes. I use a black cloth underneath my screen and a piece of regular window pane glass (no uv coating) to keep the art against the screen. After exposure, wet the screen on both sides, let it sit for a minute or three, start with a light pressure from your hose until you see the art opening, then step up to a medium pressure working in circles until your entire image is open.
Let us know what you figure out.