I'm trying to make royal icing flowers (which I've done before, which success) and for some reason, its VERY thin and liquidy. I made it exactly according to the recipe. I know that adding more powdered sugar could thicken it, but I didn't know if there were alternatives.
Adding PS is all I can think of. Hopefully, one of the experts around here can help you more.
Add powered sugar a little at a time until you get the consitency you need.
I'm guessing you didn't mix it long enough. If you're experiencing high humidity, you may need to beat it for as long as 15-20 minutes before it reaches stiff peak stage.
Another possibility is that some oil or grease got into your mixing bowl or on your utensils. Royal icing will not fluff up properly if it comes in contact with even a wisp of grease.
You can also add some cornstarch in it to help thicken it up. I use the corn starch at Michaels in my classes. Most people either bring the icing in with not enough or too much water. Stir in some cornstarch and let it sit for a while.
Also does it feel thick but doesn't hold it's shape? If that is the case, then you need to mix it longer.
Check out Antonia74's icing http://cakecentral.com/cake_recipe-1983-0-Antonia74-Royal-Icing.html . I absolutely love it & will make no other recipe. I've never had humidity affect it & it's sooooo humid here in TN