My dwarves happily dug staircases below level 100, but none of the others seem to want to go out and collect stuff from the caverns down there. They just ignore the collect and dig orders. They have a clear path from the staircases, and are happily doing the same thing from caverns at higher levels...
What am I missing?
Generally, miners and gatherers work from the top of the world down to the bottom, from the west side of the map to the east side of the map, and (seems to be a weaker preference) from the north of the map to the south. This is general - sometimes they go in a different direction. But the overall algorithm for job-selection seems pretty strongly biased in those terms.
So they will usually dig out the designations on z+10 before moving on to the ones at z+0, gather the plants on the surface before working the cavern, cut the trees in the NW corner long before getting to the SE corner, and so forth. They're not ignoring your orders, they just have other orders they're doing first.
I think there is also something of a bias for more recent orders. For construction designations, I'm sure there's a strong bias - designate a thousand walls at the same time, and the last ones designated will be the first ones built 9 times out of 10. For digging and collecting I wouldn't bet money on it, but it seems to be that way but much weaker.
Usually if I want them to do something deep, I designate that first and don't have any other designations for them until they finish that job.