I am not sure about your particular transfers but I have never heard of any that do not require parchment paper between the iron and the transfer. (with dark transfers) It may help to go back and read your directions. If you need parchment paper, you can find it in the grocery store either with the baking supplies or the foil.
Oops - sorry I misread your post - I now see you are using light transfers - and these, I believe, do not need parchment paper. It may be your iron or ironing temperature.
If you do this and still get the mark - your iron could be too hot. Your directions should state and what temperature you should be using.
I hope this helps!