Tiles are not being shown?!!?!

Hello, I am a beginner, still… I have this problem that if I make my background visible it culls the side tiles of the ground, as seen in first picture. The 2nd picture shows the ground tiles with the background not visible. The background Y is set to -1000, and so should not affect anything on Y plane 0. I’m assuming the tiles are being culled for some reason, but I do not know why? Any ideas? Please…

I have included the top view, which seems fine to me. The background is set back behind the ground tiles, but the background gives the impression it is concaved and so covers the widest (leftist & rightist) ground tiles. The player can still move along these tiles that can’t be seen, and drops off into oblivion if he walks too far. So the tiles exist, but why are they displayed behind the background? ARGH!!!