Do be sure to check the instructions on your interfacing. I use French Fuse a lot and it is put on with steam, I noticed that some I had gotten must have been older, stored in heat - something because it was had to get to stick. I use a press cloth with it and ended up having to wet and ring out the press cloth to get enough moisture (steam just wasn't enough) to get it to stick well. Even though your instructions might say 10 seconds, you should probably try a piece on scrap t-shirt fabric using different times until you get the result you want.