In this paper, we present the evaluation result of our constraint-based tutoring system for logic programming from which we derive the conclusion that students need diagnostic information and remedial hints corresponding to the stage of the problem solving process where they are stuck. For this reason, we propose a three steps diagnosis approach which consists of: diagnosis at the task analysis stage, diagnosis at the solution design stage and diagnosis at the implementation stage. Our diagnosis approach should not only help students learn logic programming, but also master the skills of task analysis and solution design.