Could you share the recipe(s) you're using? But as a general rule, a lot of people end up with runny icing because they don't beat/whisk it long enough before adding more liquid. It takes a while for all that powdered sugar to be absorbed, and it's a dry sugary mess before that, so people think they must not have enough liquid. They add more and poof, runny icing.
Another thing you might try is creaming the butter | cream cheese | shortening with the sugar and any other dry ingredients before adding any liquid (milk, water, flavoring, etc.) Then add the liquid very gradually until you get the consistency you want. And if it's still too thick, add more flavoring a few drops at a time.