There's not enough information in your question for me to understand the model. But in general a purely discrete event simulation will stop when there are no more events to be executed, or when the stop time is exceeded, whichever comes first. Since you have already increased your stop time, we can say that the simulation is stopping because there are no more events to be executed. Try using the SimEvents Debugger SEDEBUG to step through your model and see why this is the case. Perhaps entities are being blocked, a gate is not being opened to make way for entities etc. - there are several possibilities.