Dupes wont build

It would be best to show screen shots, so we can see the problem. Press F12 to screen cap.

The only thing I can surmise from your description is there is a block tile in the way barring them access. There's also the fact Dupes are too busy doing other priority 9 tasks, so when you do have them ordered to dig at 9 it goes into a queue. They have to finish all the other tasks before getting them to dig. When you re-did it it pushing it back to the end of the line. I've had it where it took 2-4 cycles before they were able to start digging.

How many other task and things do you have set to 9?

Edited April 15, 2017 by vonVile