If the car is still new (less than 10,000 miles) just give it time. Your mileage will significantly improve after break in. Unless you're keeping your foot in the accelerator, you absolutely will get the numbers posted, and in fact, you may well exceed those numbers. I consistently get 28-30 on my Impala, and have hit 32 on several occasions when the windows were closed, no a/c was needed and driving on flat geography at highway speeds.
To demonstrate, I recently purchased a new Equinox. For the first 2,000 miles I averaged 18MPG. Then it gradually started creeping up. By the time we hit 7500 I was averaging 22, and again, same exact driving (city traffic, slow speed) conditions. Now that it has nearly 12,000 miles, we average 23 in city traffic, and around 26 on trips.
Willy