I made an Easter egg cake yesterday (thanks to some great inspiration from CC!) with gumpaste orange chicken legs sticking out. The "egg" was covered in buttercream and then MMF over that. The legs seemed very solid, set, and dry before I poked them into the cake, but with a few minutes they had totally turned to mush and fell off. What did I do wrong?!
Your b/c has probably started breaking down your gumpaste.
What you can do next time is to stick either a bamboo skewers/dowel/toothpick/uncooked spaghetti into your gum paste piece and stick that into the cake instead. You have to put the skewer (or whatever you think will support your piece the best. If it's small the uncooked spaghetti or toothpick will work but if it's larger I like to use skewers) into your piece before it dries.
gumpaste can seem dry, but still be wet in the middle, so it will melt in buttercream. if they are dry right through it should be ok.