The rods have to be adjusted so the wastegate opens at the same pressure. It's on off the car job. They should have been setup, or at least checked, before they were put on. That's about the limit of my knowledge.
I'm not saying 100% that this is what's happening on yours. Can you get back to the tuner and get them to see find anything? If it's only just happened after you've had the gtss installed might be something simple. Hose left loose or a leak somewhere.