This happens (usually) when there is a collision in the are that will be built, I couldnt build upper frame on a watchtower because a small tree branch was in its path, another example is when building stairs they wouldnt build because there was a peice of paper on the ground where the stairs area is,... Devs we would much rather being able to build in more places on slopes etc, and very close to other items buildings, trees, rocks etc... please also look at tickets T141391 and T141393 these also need addressing.