Slow fill of GTO gas tank

I have a 2006 Pontiac GTO. I cannot fill the gas tank most times. On trips out of California, it fills easily, but in California, I cannot fill the tank, or if I take 12 to 15 minutes, I can get up to 15 gallons into the tank (holds 18). The dealer says there is no problem. The difference is that in California, there is a vapor recovery hose that clamps over the filler tube while in Oregon and Washington, for example, the gas nozzle has no such device. I have read other models that have this problem, but they all seem different. I am about ready to sell this car back to the dealer.