Mentoring programs focus on providing effective mentoring to mentees for a successful career in academia. However, the effectiveness of mentoring relationships cannot be guaranteed despite the best intentions and efforts of the mentoring program management. Literature suggests that the effectiveness of mentoring relationships depends on multiple factors. The present study explores the factors that affect communication in mentoring between mentors (professors) and mentees (postdocs) in higher education. A qualitative study was conducted in universities offering mentoring programs for postdocs in Germany. Twenty-five in-depth semi-structured interviews were conducted with fifteen postdocs and ten professors in mentoring programs. Charmaz's Constructive Grounded Theory was used to collect and analyse the data. The findings show a complex interaction of mentors' and mentees' intrapersonal factors, interpersonal factors, and external factors affecting communication in mentoring, hence affecting mentoring relationships. These findings may help improve the mentoring experience for mentors, mentees, and organizers of mentoring programs. The study recommends a longitudinal investigation of mentoring relationships to understand the long-term impact of factors affecting mentoring relations in higher education.