We report an effective retrospective group discussions with Ask-Why in KWS retrospectives as a way to enhance problem solving skills. The KWS retrospectives has two frameworks: KPT and Ask-Why. We have applied the Ask-Why to group discussions in engineering design education. During the Ask-Why, students hold discussions with 4M analysis to work out the reasons for what went wrong in given scenarios. Once the root cause is identified, the students design multiple solution strategies and devise short-term and long-term measures to be implemented. As a result, we conclude that the group discussions using Ask-Why is able to demonstrate an educational effect in terms of the development of problem solving skills.