3D printing has numerous applications and photopolymerization is one of the main approaches. However, its products have limited mechanical strength – why? In this article, Gojzewski et al. use AFM to identify and quantify the weak spots in polymer blocks created from acrylate-based resin with layer-by-layer photopolymerization in a commercial 3D printer. PeakForce QNM modulus maps find a 25% decrease in Young’s modulus at the layer interface. High-resolution maps show that nanoscale morphology and chemistry remain unchanged, and the stiffness decrease is attributed to lower cross link density caused by the presence of oxygen, which inhibits photopolymerization. These results suggest a route to optimizing 3D printing for improved mechanical product performance.