I don't stay full for long...?!

I make sure to eat breakfast every morning. I usually have an apple, yogurt, another type of fruit and a couple of nuts for protein. Then I eat those all over the course of 1 or 2 hrs in the morning. About noon I'll have a half romaine/half spinach salad with carrots, tomatoes, radishes, onions and a bit of parmesan cheese and then I put two Tbsp of ranch in a separate cup and just dip a tiny bit on the fork before taking a bit of salad. So the salad is fairly large for approx 300cals. Yet within an hour of eating it I'm usually hungry for something else and I feel like it should keep me full longer since all that healthy stuff is in it? What gives?