Your mixture may be lumpy if you did not allow the cream cheese to come to room temperature before beginning. Be sure that your cream cheese has been well softened. Take it out of the fridge at least 30 minutes before making this recipe. Remove it from the packaging and cut it into pieces and set them so they aren't touching each other. Do NOT use the microwave to soften your cream cheese. If you find that your lemon cream cheese mixture is lumpy, switch your mixer to the whisk attachment and beat until lumps are gone. Bringing the milk to room temperature is also a good idea and will keep your cream cheese from lumping.