After the boring machine head gets X distance away the only thing it finds is cobble. I assume this is some problem with chunk generation, only thing I can do to fix it myself is travel ahead of the boring machine, generating chunks the old fashioned way.
This happened to me as well, I looked in the source code and this isn't intentional.
The code for the Boring machine is really ugly and it looks like it would thrash the chunks a lot. I have no idea why it only finds stone as opposed to air. When you fly ahead of it do you see chunks with no ore generation where it dug?
It also seems to get less ores than it should, although it might have been a glitch in my setup.
With a silk touch bore full size I got 37 certus quartz (at the right level) after boring a LONG way.
When I flew through the cave and gathered the quartz on the edges I counted 50.
So the bore found less quartz in the large center than I did on the edges, which is statistically unlikely.
I also seem to be getting less of everything else compared to memories of redpower tunnel bores of similar size going the same distance.
There is nothing in the source that nerfs ore collection, there are no distance nerfs, there is nothing suspicious whatsoever.
One thing I did figure out is that the Boring Machine can eject into chests, previously I was collecting the item entities it spit out.