One thing I noticed is it takes quite a bit of time for the transfer 'ink' to transfer to the fabric. The iron has to 'sit' on the spot for at least 30-60 seconds, if not longer. Just be careful not to scorch the fabric. Every so often I check by carefully lifting up the corner and if the transfer isn't dark enough for me, I put the iron right back on that spot. I do that for the entire design.
You might want to try tracing paper or newsprint. It's thinner than copy paper and should get hot faster since it isn't as thick.
Your other option is to tape the pattern onto the window pane, tape the fabric over it, and trace the lines with a pencil. Unfortunately this only works with light fabric.