Why do my mirror cakes lose their shine after a few hours?

Maybe its the recipe you are using?  How long is it taking to loose the shine?  I have only made one but it was still shiny 4 hours later.  After that it was gone lol. 

I used water, sugar. Swetened condensed milk and gelatin.  One of the cakes was just cold when I poured the mixture over it and the other was frozen cake.  I finished the second cake at about 7 pm and when I got up about 6 am it had lost its shine.

 That's so strange,  that sounds like the same recipe I used.  Im not sure why it would lose its shine. I hope someone comments that might know.  

Thanks, me too.  I want to do it again but I don't want a third shine loss!