Why am I not getting bigger shoulders or deltoids?

I lift weights 6 days a week for and hour, on the days I workout my deltoids (shoulders) I do it for an hour and a half, I workout my rear, middle and front deltoids. My rear and front deltoids are coming along fine but my middle deltoids don't seem to be getting as big as I want them and barely seem to be getting bigger. I drink whey protein shakes after lifting. I want those deltoids that you see on tv where they're sticking out of the top of your arm but it's taking forever for me. I always lift till failure with workouts like holding the weights in front of me and pulling it as high As I can, I also do the ones where I lift them to the side of me and some others. Anyone know if I'm doing something wrong or know anything else I can do or wanna share any tips, thanks