Some slab intersect and some don't... I need them to look smooth instead they show lines

[cocoloco]

Hi! Could someone help? I am building mantel fireplace. I am using ONE material of slab, and all is aligned. all is set up same except different heights. as you can see in photo. The upper mantel DOES NOT intersect (it is showing line...) which I hate. I tried everything.... and then I did mantel under it, and it dit intersect. And that's how I want all these slabs to look like.  I don't know what is different because as I said all slabs are same settings except the heights. 

PINK = showing line and I don't want that

GREEN - not showing line and connects properly. that's how it suppose to be. 

Thank you

Operating system used: Mac Intel-based

 

[vistasp]

Is it possible that the front face of the horizontal slab is not exactly in line with the vertical supports? Even a fraction of a millimetre difference will cause a line to show up.