I have no experience using storebought ricotta, I mostly only use ricotta when I've been making cheese and can make whey ricotta. Clumping together rather than coating the potatoes sounds fairly normal to me. But I recall reading somewhere that the ricotta sold in the stores quite often has cream added to it, and that for some recipes the ricotta needs to be drained (mine doesn't). So that makes me think that your whole milk ricotta is a lot more dry than the supermarket variety. Maybe try mixing in some milk or cream after draining the cheese and see if that makes a difference.