Single-layer monochalcogenides have recently attracted much attention in the scientific community for their strong non-linear response, in particular the second harmonic generation(SHG). The origin of this strong response is still debated. A strong mismatch exist between theory and experiments. In order to clarify this issue, in this work we performed first-principles calculation of linear and non linear optical response of GaSe and InSe monolayer beyond the independent particle approximations. Our results show that correlation effects do modify optical response functions, but they are not able to explain the experimental findings. We discuss possible origin of this disagreement between theory and measurements.