I sympathise schroeder the same thing happened to me when I made red velvet cupcakes for the World's Biggest Coffee morning. Apparently I shouldn't have bought the posh cream cheese but gone for Philadelphia.
Firstly, are you using a bit of butter as well? Also, use full fat cream cheese, low fat does go runny. Finally, don't over beat the cream cheese, just a quick beat to soften with the butter then straight in with the icing sugar. Over beating also makes it runny. HTH
schroeder all the other recipes I've seen say 2 parts cream cheese to one part butter, then 2-4 parts icing sugar, with a little lemon juice/vanilla extract. Keep adding icing sugar until it's thick enough.
So try adding another pack of cheese, and lots more icing sugar and you should get there eventually!