Purpose: Developing graduate students' academic writing has been a major concern for many scholars over the past few years. Existing literature on the challenges of thesis writing has not focused on master's students in English language teaching (ELT). Data on the challenges have been mainly gathered from the theses and focused on the structure and genre requirements. Few available studies have investigated such challenges through the lens of supervisors in an international context. Knowledge about the factors hindering the thesis writing process of non-native MA students in English from the supervisors' perspectives is scarce. This study attempts to fill these gaps by answering this question: From supervisors' perspectives, what factors hinder the thesis writing process of non-native MA students? Design/methodology/approach: Thirty supervisors from state and private universities across Iran voluntarily participated in this qualitative study. Drawing upon teachers' diaries and semi-structured interviews, the authors identified major factors negatively influencing the thesis work of master's students. Data were transferred into NVivo 10 and analyzed thematically following Colaizzi's method. Findings: The study found that factors constraining students' writing were (1) students' lack of effort, (2) students' lack of a strategy for writing, (3) students' lack of autonomy and (4) students' absence of voice. Originality/value: The authors discuss the practical implications of these factors for different stakeholders. There is a growing interest in postgraduate students' thesis writing processes. Surprisingly, no research exists on supervisors' perceptions of factors that constrain the thesis writing process of non-native English master's students.