You'll have to be more specific in your question. What physical mechanism do you think is at work when the wire formation fails? What type of material is the flux? Could this be a fluid flow problem? What type of fluid would you consider your flux to be?
This may not even be a problem you need simulation for. You could examine the flux extruder and see if all its parts are working correctly. Is there something preventing it from extruding (or forcing) the flux at a steady rate?
I hope that this could help you out somehow.