With student-centered, hands-on approaches to real-world problems, integrated STEM (science, technology, engineering, mathematics) curricula have the potential to engage students more authentically in multi-disciplinary learning and to learn mathematics more deeply. However, researchers have shown concern that the "M" is often a silent partner and have called for increasing mathematics' visibility in STEM curricula. Taking into account this background, this paper investigated how teachers' and students' engagement with mathematics within a specific lesson in an integrated STEM unit evolved over four years in two elementary teachers' classrooms and the factors the teachers pointed to that may explain their adaptations. Using classroom observations and teacher interviews, the results showed that the time engaged with the mathematics and the depth of engagement with the mathematics decreased over the four years. Five themes emerged that may explain their adaptations: 1) limited time, 2) the need to address required school academic standards in limited time, 3) materials and preparation, 4) the teachers' comfort with the curriculum and, to a lesser extent, 5) their perceptions of their students' abilities. These findings have implications for those that are writing, studying, or potentially using integrated STEM curricula in elementary schools.