Unfortunately there is often a huge quality difference between genuine exhaust systems and non-genuine ones. The latter often don't sit correctly, poorly manufactured, and need heating up and tweaking here and bending there. It's always been the case for as long as I can remember having worked in the trade for over 40 years. I suspect you have a non-genuine system that doesn't fit exactly right. I'd go back to the people who fitted it and say you're not satisfied.