Possible causes and solutions
The chocolate you used was too fluid
A three-drop chocolate has exactly the right cocoa butter content to produce the perfect end results. Chocolate with more cocoa butter in it will create a sandy texture.
The cream was whipped too stiffly
Always whip the cream into medium peaks (i.e. 2/3 stiff) and not more. If it’s too stiff, you can’t fold it in easily, creating a sandy texture.