It is not the griddle it is the stove. I have a round cast iron griddle, that fits on one burner, that I use all the time for pancakes, french toast, grilled cheese, etc. I use an electric stove and the burner can have hot spots, and as earlier stated you do need to let it heat until a water drop "dances". You can see where the hot spots are when you boil water and how the boil starts in the bottom of the pan, it usually is not even, especially on an electric stove.
If the griddle is a large one that fits over 2 burners, where ever the heat is not under the griddle, the heat will not transfer the same as over the direct heat.
Hope this helps.
PB from MN