The simulation tool was applied to investigate the reasons behind the anomalous reduction in the injected mass, which was detected in a direct-acting prototypal injector at medium and high loads, as pnom was increased under fixed ET. The time interval (NOD), required by the piezo-actuator to recover the gap between the push-pin and the balance plate as well as the nozzle-axial deformation, was shown to increase significantly with pnom. Furthermore, the increased deformation of the lever-amplifier with pnom reduced the amplification factor with respect to the design value C0. Since the current profile supplied to the injector for fixed ET was the same at the different pnom values, the piezo-stack lift reduced with pnom and, as a consequence, the needle could not reach the end of its stroke.