I am making cupcakes and using leftover crusting buttercream. The cupcakes have sprinkles but they just roll off even if I put them on immediately. What can I put in or on the icing to make it sticky?

You may want to add more liquid to the icing or very, very lightly spray the finished cones with water and sprinkle--there is the risk that the colors of the sprinkles will bleed, so use as little water as possible. Or, re-whip your icing with a bit more liquid and sprinkle them quickly after icing them. Good luck!

If your buttercream crusts that quickly then I would add the sprinkles each time you frost a cupcake and not wait to do it all at once.

I have the same problem with dragees..
So the best thing to do as carmijok said is to sprinkle them immediately after icing & dont wait to do them all together...
Another thing you can try is using a non crusting BC recipe