Unreachable Dig is a problem again

Maybe you have a locked door or deconstructed some tiles that were needed for dupes access. Lots of other possible issues. Without a save file, these possibilities are impossible to eliminate. 

yeah, nah. Problem solved by reloading. Every time I saw it was when I planned multiple constructions on the same raw tile.

I keep getting that very often, but I don't know what triggers it.I have to cancel the order and replace it to get it working. Quite annoying to realise after a long time that the duplicants never started working on something they could have. :/