Smoothie question and the Vitamix mystery.

I was making the simplest blueberry smoothies when I started noticing the smoothies were too thick after blending for 30 seconds. I had to use a spoon as it thickened up quickly. And I don’t know why. I stopped with the blueberries and now use strawberries with no issues.

Recipe 1 cup fresh blueberries, 1 small banana, 1 cup whole milk.

Any reason why it’s so thick?