Clearly, insufficient light is a factor. But you haven't provided your history of repotting, root pruning, or dividing your plants. No plant can thrive for years without tending to the root system.

Are you growing them indoors? I'm in central Alabama and grow mine outside. They can take a fair amount of sun, but will do better with afternoon shade. Rhizo's right, if they aren't getting enough light, they aren't going to grow. They won't die (hence the name cast-iron plant), but they aren't going to grow.