Snowdrops

Every year this tiny clump of snowdrops comes into bloom and, along with an even smaller clump nearby, they are the only naturalised ones in our garden. They never seem to multiply and despite my attempts to plant more, they never seem to survive. It seems strange and the only reason I can think of is that the mice and voles that inhabit areas of our garden probably feast on the bulbs. But why don’t they eat these ones?