A "lollipop" serves usually happens due to hitting "up" on the ball and not hitting it behind or on top of the ball.  What I mean by hitting up on the ball is  after you throw the ball up for the serve you dont allow it to drop far enough down before making contact.  It also happens sometimes when you hit the ball with the heel of the hand instead of the palm.  Another reason it happens may be to how high you throw the ball up, if the ball is not high enough on the toss it wont allow you to fully extend the serving arm which will also cause you to hit under the ball making it lollipop.