Building my base, but the floor pieces won't "snap" together.

[Check Six]

The ground is flat (I even tried raising and lowering the piece that wouldn't snap. I've tried "R", and demolished and rebuilt several times, it just doesn't seem to work. Am I doing something wrong? I've watched a BUNCH of building vids on YT, and it just snaps easily.

[PureHTDz]

Make sure your using the floor piece and not the roof piece , they look exactly the same.Any terrain that is in the way of floor pieces stops you placing them or snapping. The terrain has a large hit box which you can not see even if the terrain looks flat, so i work around this by placing elevated floors, you then have no snapping issue and enemies can climb them either, even if they are knee high.