Had a problem with my cake, A Fruit cake using Baking Soda,Baked well, when decorated by first,  Brushed with Jam, then covered with Marzipan and Fondant, after 2 -3 days the Marzipan started melting and flowing out of the cake.  Can someone tell me WHY?   Where did I go wrong ?

Did you wait for the marzipan to dry for a day or two before covering with fondant? That may have been the problem. But even if you didn't, "melting and flowing out" would suggest a serious hydration issue, too much alcohol, too much jam, bad marzipan? Did you roll out the marzipan on cornflour? That can cause fermentation of the dried fruit and I shudder to think what might be happening inside the cake if that happens.