N.K.
Wow, that's interesting... Two things come to my mind:
1) Maybe your freezer is not working properly
2) Maybe you got too much sugar or alcohol in your sorbet
"If too much sugar is present, the sorbet will not freeze properly.
Read more: http://www.answers.com/topic/sherbet-and-sorbet#ixzz1LcxS...;
Are you sure you do not have any organic solvent in it? :-) I work in a chemistry lab and water solutions mixed with organics like acetone and glycerol do not freeze at -20C. As JoG is saying, maybe the cocoa oil in chocolate is preventing it from freezing.