It didn't fell because it is still attached to surrounding ground, rocks or trees.

You have to figure that the deposit you see on screen is an assembly of models built on invisible cubes. You have to clear materials around those cubes enough, like one meter or a tad more.

This can be impossible to do sometimes, when the deposit reaches the max depth you can dig. In this case, try to access the deposit from below, from another entry point of lower elevation (you can only dig so deep from the original ground level).