They never would have started the project unless their equipment had the capability of grinding its was through igneous rock. That's what makes this story so intriguing.
What could they possibly have run into that's too hard for Bertha to grind through? Or is it that they don't want to damage whatever it is they've found?
Probably just a matter of having the right cutters. Sort of like the difference between a wood and metal saw.
I have no experience with continuous miners but they are drilling through a type of conglomerate which is difficult due to the various hardnesses of the rock making up the conglomerate. If there is a rock of considerably greater hardness on one or the other sides of the drift, I can see problems. Something unanticipated anyhow.
The hardness isn't the issue. It is thought that the object -- probably a boulder -- won't stay still. It moves a little because of a void or soft earth behind it and so the cutting teeth can't get purchase.