let me refer you to this thread I made:
http://discourse.stonehearth.net/t/dev-3010-hearthlings-refuse-to-mine-stairs-other-things-near-the-ground/23088/2
the hearthlings can only mine blocks within their reach, if you imagine this is a 5x6 wall of blocks this is what they will mine,
[][][][][]
||xxxx|[] 2 deep
||xxxx|[] 3 deep
||xxxxx|| 2 deep
||xxxxx|| 1 deep
[][][][][]