Sometimes with a product like that you have to spend a good bit of time ironing with some good downward pressure. Start from the inside and work your way to the edges. Leave the iron on it for longer than you normally would for just fabric only.
I used some kind of fusible product as a stabilizer for a purse. It took a lot of muscle and patience to iron it on. The edges didn't stay perfectly fused, but I knew that they would be part of the seam allowance anyway.