Super late, but...

If you're building upwards and stacking, try taking one of the previous foundations out then try to place the one giving you trouble. I found that this typically works when I have this issue.

ok thanks for the tip

I find it's the triangle foundation blocks I have the most trouble with. Many times I have tried to set them and sometimes they don't even want to snap in. Other times they snap in and turn green but then they just wont build. I had one block that was crucial to my design that no mater what I did I just couldn't get it to build. I removed blocks, I added blocks, at one point I had disassembled a whole section of my keep right down to the ground and then built it back differently but that one block still wouldn't place. I finally just had to build it differently.

Another thing I have found is, when I have floor laid out and suspended out over a cliff or a drop-off, I can then attach square foundation blocks underneath them but not the triangle ones. This is very frustrating.