Quantitative reasoning (QR) is a key skill for undergraduate biology education. Despite this, many students struggle with QR. Here, we use the theoretical framework of student noticing to investigate why some students struggle with QR in introductory biology labs. Under this framework, what students notice when given new information and data influences how they process this information and connect it with other events to form new conceptions. Students must mentally isolate given features, create mental records of those features, and identify features or objects that they connect to existing knowledge. Identifying these features or objects is thus critical since they form the foundation upon which learning takes place. We conducted observations of groups in introductory biology labs involving QR, which informed follow-up interviews to examine what students notice, the level/relevance of their noticing, and factors that shape student noticing. We find that some students are noticing more perceptual features, often focusing on less relevant trends and features, with others noticing deeper, more relevant patterns that facilitate conceptual sensemaking. In addition, we find multiple factors, including students' expectations and their attitude toward QR and biology, that shape student noticing. We conclude with implications for instructors and the biology education research community.