Quick question - maybe some of you know the answer......
Sometimes when I pipe onto a fondant cake with royal icing, it will look fine initially but then later start to "melt" (for lack of a better word). For example, a piped pearl border might start to sag. Or a decorated design might slightly slide down the side of the cake. Not completely, but just enough to look sloppy. Why does this happen? And why does it happen sometimes, but not all of the time?

I make my royal icing with fresh egg whites, and enough powdered sugar to make it a little thicker than I need. Then I thin it just a bit to a pipeable consistency with a tiny bit of lemon juice.

Any ideas? Do you have a "fail-proof" royal recipe different from mine? Thanks for your help!

You are using the right ingredients. the only thing that comes to mind is that some grease got into your icing - has happened to me. I wipe my bowl and beater with a bit of lemon juice or vineagar to be sure I have no grease.
Good luck.

how do you roll your fondant out? Do you roll it out on a thin layer of vegetable oil? That grease will break down your royal and not allow it to dry.

It sounds like you aren't getting the icing whipped up quite thick enough...or maybe when you are thinning it, you are getting it slightly too thin. It will sag if it is not whipped nice and stiff and matte (not shiny).