Help everytime I make Royal Icing, it starts out fluffy but after 24 hours it turns out runny. What are some possible reasons why it's falling flat.

Hi, so that we can get a better picture of what's happening, could you tell us what recipe you use, your method for making it, and what you mean by "fluffy"?  Royal icing should be smooth and stiff.

I've heard that if you get grease into a batch of RI then it will fail, but I would have thought that would have been an immediate result rather than over night.
 

Check to make sure that none of your flavorings have oil in ingredients listed on the label. Been there, done that!

I think it was an oil problem. I made a new batch and before I started I wiped everything down with vinegar and I didn't use any flavoring. So far no problems. But thank yall for the helpful info.