u must be unlucky, i´ve baked 100´s of fruit cakes in my life, and the fruit has never sunk to the bottom. A typical cake of mine would consist of eggs mixed with sugar , butter and spices, then i´d fold in the flour gently. grease and paper the cake tin, spoon in the mixture, and bake the cake.
Is the mixture prior to adding the fuit quite soft/runny? There could be two reasons for this.
I've found that when I use margarine instead of butter, the fruit sinks. Also if the eggs are too big, it can make the mix too soft. I always use medium eggs, but they can vary in size quite a lot, so I try to use the smallest I can find.