Why did my jello not set? To a 3 oz. box of orange jello I added 1 cup of boiling liquid (mandarin orange), then a pint of orange sherbet and finally the drained oranges.  The jello had a bubbled effect on top and did not set well.  Vanilla ice cream worked just fine; do I need to change something in order to use sherbet?
A lot of fruits denature gelatin so it doesnt set. 

All of the citruses, papaya (that has an enzyme called "papain" that breaks proteins), pineapple ("bromelin") and a lot of others cannot be used with gelatin without being very thoroughly cooked, as in boiled for one hour or so. 

If you just brought mandarin oranges to a boil for using, that could be it. Or it could be the sherbet.