In recent years, product development is becoming increasingly complex. The ability to continuously learn from experience and improve existing business processes has become an important competitive strategy for many manufacturing companies. Many companies have established lessons learned practices as one of the key knowledge management efforts for the collection and dissemination of experiences in the form of lessons learned. However, according to the literature, many companies are still struggling not only to collect meaningful lessons learned, but also to reuse them in new situations. To address these challenges, a research study has conducted in the aerospace industry in collaboration with two manufacturing companies. The purpose of the study is to identify the potential barriers in the lessons learned practices in the context of a complex product development. The study identified several challenges in the lessons learned practice, which were mainly grouped into ten barriers. Finally, based on barriers, this paper outlined relevant recommendations for improving lessons learned practices in complex product development environments.