The dark transfers generally have to be peeled off a backing, too. Read the instructions very carefully all the way through before you begin. The very first time I used the dark paper, I didn't read the instructions thoroughly enough. I was used to using the light paper. I missed the peeling the image from the backing part, and it didn't adhere to the shirt. That is probably what happened to you as well. Peeling the image off the backing isn't always an easy task.
Beth