If you're finding that your Ninja CREAMi sorbet is crumbly, there are a few potential culprits. One possibility is that the machine isn't being operated correctly - specifically, that it isn't being shaken enough. According to the Ninja Kitchen user manual, you should shake the CREAMi for 60 seconds before serving in order to evenly distribute the ingredients and prevent clumping.
Another potential cause of crumbly sorbet is using too much or too little of one of the ingredients. The recipe for Ninja CREAMi sorbet calls for 1 cup (250 ml) of milk, 1/2 cup (100 g) of sugar, and 1/4 cup (60 ml) of cream. If you use more or less than these amounts, it can affect the consistency of your sorbet. Be sure to measure out the ingredients accurately before adding them to the machine.
Finally, if you're not eating the sorbet immediately after making it, make sure to store it in an airtight container in the freezer. Sorbet can start to degrade and become crumbly if it's left out at room temperature for too long. By storing it in a sealed container in the freezer, you can help keep your sorbet fresh and prevent it from drying out or becoming crumbly.