Smoothies are made thick by fibrous fruit. Bananas are so popular because they're sweet, and they're a great thickener, but I prefer using fruits with a less unique flavor. Pineapple is a great thickener, as is mango, peaches, and avocado. One reason that your smoothie may not be as thick is because of your blender. A weaker blender is going to need more liquid in order to properly blend your smoothie, and that will result in a thinly textured smoothie.