Honestly I don't think anything is obstructing these things, I think they're just buggy. A bit like the "terrain is too uneven" message when putting paths on slopes. Sometimes it's true, there's something there that you can fix, but sometimes there's nothing and you have to erase what you're doing and start over as a workaround.
At one point I was trying to build an indoor viewing gallery. I had the path aligned to the grid, which was lined up to the barrier perfectly (used a grid-piece as a guide for the fence), and the game was happy to let me place the path along the whole length of the barrier except on the very corner. I removed the fence out of the way, put the path down, and then I was able to move the fence literally back to the exact same spot (I added the fence back in using angle snap, so I know it was the exact same spot). It's very annoying. If I can put the barrier against the path, why can't I put the path against the barrier?