Are you building near the ground? The parts get more red/less structurally sound the farther off the ground you are. Are you building on top of parts which are on the ground? Is the ground flat under your floor and supporting it? If a floor part is green, it's not on the ground; if it's blue it's on the ground. I think you get greater structural integrity if your walls/supports connect directly down to the ground, not to things suspended above it. But it's hard to tell just from a description!