Hmm, I think I just found a bug with digging down. Has anybody else had there miners just stop digging and say they have "No job" when you dig down far enough? I had a solid up/down stair case going for 33 levels (Got lucky and hit a pillar in the cavern) and suddenly they don't want to know about level 34.
Yeah, I've had this bug a few times. Do you have any flowing liquids on your map, like chambers that are slowly being flooded/irrigated? Twice I've run into this problem with my miners, they wouldn't get to work, apparently the area around the mining designation is deemed 'impassible' for some reason. Both times I had different flows, once was my farm area being slowly flooded, another time was magma draining down into an underground chamber. Both times the mining designations were no where close to these flows.
Another possible issue is that it could be that you've dug PAST the first underground cavern and are now in the 2nd Underground Cavern. In my experience, for some reason this screws up job designations, especially when trying to collect webs because they'll want to path through the 1st cavern to reach the 2nd, even when the 1st is undiscovered.
Start trying to dig to level 34 in other areas away from the first staircase. I've also had luck designating ramps instead of stairs when the miners start canceling jobs for no good reason.