Although improving instructional supervision has already been featured in theoretical debates and even advocated for in the policy frameworks of many nations -- including the USA, UK, and countries across Europe -- little is known beyond the Western world about the potential difficulties faced by school principals seeking to implement instructional supervision practices to promote teaching and learning in their schools. This study investigated barriers to school principals' effective instructional supervision practices. The participants of this qualitative study included a total of 57 principals and teachers working in secondary schools. Data were collected through interviews and analysed through qualitative content analysis. Bringing evidence from a centralised educational context, we identified four major barriers to the effective instructional supervision process: (i) lack of supervisory skills and content knowledge, (ii) negative attitudes to supervision, (iii) maintaining a family-like school atmosphere, and (iv) lack of time and heavy workload. Based on the findings, we provide several implications for policy and practice.