There are a few ways to get grainy potato soup.  1. It's overcooked. If you boiled the potatoes too long in the broth, they may become grainy because they've absorbed too much moisture.  2. The skins of the potatoes weren't removed. You might not have grainy potatoes, but just pieces of the skin that don't puree as well as the inside of the potato. I like to peel my potatoes for this dish.  3. Wrong potatoes. Different potatoes can have different starch levels, and using a starchy potato can produce a grainy potato soup, while a waxy potato will have a smoother texture when blended.   4. Freezing. Freezing can give this a grainy texture that is unpleasant. I don't recommend it.