Ok...here goes. Is it humid out? The humidity will change the consistancy. You have to use a metal bowl when mixing it. Plastic bowls that may have been used with shortening make the royal icing melt. Same goes with tips. Boil your tips. Any speck of shortening will make your royal icing melt like butter. I mix my royal icing until the sheen goes away. Which is about 8 mins. with my kitchenaid mixer. It may take longer with another brand of mixer.
My teacher told me that with Royal icing...you just need to pratice making it. She said most people who are making it for the first time, seldom get it right the first try. So don't be discouraged. It took me three tries before I got it right.
Hope this helps!