I made this cake...

It's covered in BC and the decorations are MMF. They  started sliding down the side and I think the only reason they didn't keep going was because the piping on the bottom stopped them. They are a little on the thick side. Could that be why--too heavy? Should I have used something to attach them (I just stuck them on)?

Any time I attach MF things to buttercream I make sure that I roll the fondant as thin as possible and make the buttercream thicker then I normally would.  It's a little harder to get the buttercream super smooth, but making it 'stiffer' keeps the fondant pieces from sliding.

Cute cake by the way

Thanks! Is it better for the cake to be chilled or room temperature when applying the pieces?

I agree with mommyb.  The colder the cake the better off you are when placing candy pieces on the cake