This is the single most confusing situation to encounter as a newbie.

[weird tolkienis]

Just my opinion:

You have your "digger" Dupe, but he also has the unconstructive trait, which means that certain terrain types, if you build in a "harder" terrain type, he just won't build there. This just tripped me up, thinking "uh oh another bug". You plop down a  building anywhere, and your dupes will build but in certain terrain types, digging out the necessary space, since your dupes will do simultaneous tasks (building and digging as long as they have the skill level to dig in that area).