First, make sure that you have refrigerated the sorbet mixture for at least 3 hours prior to churning it in the ice cream maker. If you've done that and it still isn't freezing, place the liquid sorbet mixture in the fridge and allow to sit overnight. The mixture needs to be fairly cold (around 40-42°F) before you place it in the ice cream machine. Also, depending on the make and model of your ice cream machine, you may need to churn the sorbet for longer than 30 minutes.