Help with passion cake

[donna123]

I make a lovely passion cake (even if I do say so myself!) but can never get the icing right.
When I add the icing sugar (3oz) to the cream cheese (4oz) it just goes all runny. How do I make it firm up?

Stick it in the fridge??

Well, it's been in the fridge for 7 hours and it's still not firmed up! Any other ideas?

Not enough icing sugar.