As suggests, you should increase the size of the domain. It is important to include the whole nozzle, plus a zone such that the pressure at the outlet is of the order of ambient pressure. It is also a good practice to include a boundary layer on the walls of the nozzle, in order to predict accurately the separation point that leads to the generation of shock waves in the diverging part of the nozzle (as you may know, this happens in some interval of the ratio between stagnation pressure and ambient pressure).