Lings stopped constructing building

I created A large costum building wich has started construction but suddenly A few spots are flashing red but I do not know why, for the construction was accepted in the design fase, plz help.

Imgur: The magic of the Internet.

They’re red because people can’t get there. I don’t know what to do if you can’t wait a while without doing anything.

Sometimes this happens - if you manually place ladders on the red blocks they’ll manage to go there and build them (and you can order the ladders to be removed later)

I’ve helped someone with similar issues and provided some nice screenshots, let me find a link here…

[Edit]
Here, after the second pic on this reply: