low boost 6nz

Had my motor turned up to 550 and it still will not boost above 28psi. The truck pulls ok but not like I think it should. I pulled the grapevine at 40mph with 79800 gross but I've heard it should do better. I've only had the truck for 6 months and it only has 550000 miles on the truck. It just had an inframe when I got it so im worried that something is wrong. Had the truck dynoed before i bought it and at 475 it put down 410 to the wheels. I get good fuel mileage around 7 since I've had the truck but I'm just wondering what do all of u fellas think.