I make a cake and covered it with ganache. I then added some chocolate decorations on top. About 30 min later, I noticed my decorations softening. Help!

Chocolate decorations shouldn't melt from anything but heat. What type of chocolate did you make them out of? Unless it was candy melts, you may not have tempered it correctly - leading them to be ok if you chilled them to harden but not ok at room temp.

I used Ghirardelli chocolate chips and melted them in the mircowave. I made my decorations and chilled them in the refrigerator. I stood them on top of my cake over the ganache and then they began softening.

I did make the cake another time and let the ganache harden first. I believe this solved my problem.

Chocolate chips should not be used for this type of decoration for this very reason. They are designed NOT to melt (think of a cookie) and to stay a bit soft after heating (again, think of a cookie).

You can use "real" chocolate (couverture) but then you have to temper it. Candy melts (aka almond bark, Merkins Wafers, Apeels) are designed for exactly your purpose: easy melting and re-hardening.