My 9x13 cakes only get 1/2 to 3/4 inch rise on them. I'm just using a basic box mix no extender this time. Air bake pan @ 350 for 28 min.
Not enough mix in the pan. the 13x9 needs about 7 cups of batter. Cake Mixes generally make 4-6 depending on the brand and the flavor.
In the past BC was in the 4-5 cup range with white being at 4. They have recently changed the size of their mixes to a smaller amount.
Phils would make about the same about as BC did.
DH makes about 5-6. White being 5, chocolate 5.5 and the only one I knew of that made 6 was the golden butter.
So you need about a half of a cake mix more, depending on the flavor or to use something like the extender recipe.
For the reason Texas Sugar stated, I always use 2 cake mixes in my 9 x13 pans. It rises over the top where I am able to easily level it even with the pan to make a nice tall layer. Extra scraps are always welcome in my house!