This is my favorite icing which always crusts. Today it won't crust, so I must have made some small error in measurement that is causing the ratio to be off and then it not crust. But of course, I made 9 batches yesterday before I found the problem today. 
Any ideas out there as to what I should add to the icing to make it crust. Please help.
2 cups shortening
1 stick butter
2/3 cup heavy cream
3 tsp vanilla
2 pounds powdered sugar.
TIA - I'm desperate
Thanks - I think that'll do it.
Also, I made a mistake when making the icing. I used 2/3 instead of 1/3 cup whipping cream. So, if I add more sugar, it'll change the sugar to fat ratio, so I should get crusting. Thanks so much for the help. 
I wasn't thinking too clearly earlier whilst in the midst of panic.
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