Reading ability is foundational to school learning and a core indicator of educational quality. The Progress in International Reading Literacy Study (PIRLS), conducted every five years, offers a comparable picture of fourth-grade students’ reading literacy across education systems. Iran has participated since 2001; however, its average achievement has remained below the international benchmark (500) across cycles. This study analyzes barriers to reading literacy development in Iran using PIRLS evidence from 2001–2021 within a mixed exploratory (sequential) design. First, secondary PIRLS data were examined to describe Iran’s performance gap relative to the international mean. Second, in-depth semi-structured interviews with 12 education experts were analyzed through thematic coding to identify key constraints. Third, drawing on the qualitative results, a questionnaire was developed and administered to 147 fourth-grade teachers in Zanjan (public and non-public schools). Quantitative data were analyzed in SPSS using descriptive statistics, Pearson correlations, and multiple linear regression to estimate the relative contribution of content, orthography, and bilingualism to reported instructional practices, and to support mixed-methods triangulation across phases. Findings converge on three interrelated domains: (1) partial misalignment between the primary curriculum and Persian language textbooks and the meaning-oriented, text-based approaches emphasized in PIRLS frameworks; (2) limitations in preservice preparation and sustained professional development for evidence-informed reading instruction; and (3) challenges linked to bilingualism and linguistic diversity in non-Persian-speaking regions amid insufficient instructional support. Policy directions include revising primary curriculum and textbooks toward comprehension-focused pedagogy, institutionalizing scalable teacher professional learning, and adopting linguistically and culturally responsive supports for bilingual learners.