I used soy/almond/oat alternative in my LatteCrema Cool carafe and the result in cup is not good. Why?

The result in cup can vary according to the milk or plant-based drink used as plant-based drink formulas might vary a lot from brand to brand, from type to type, as the list of ingredients often changes.
Make sure your milk (or plant-based drink) is at fridge-temperature before using the LatteCrema Cool carafe.