Wrapping paper seam issue

[Holton_art]

I have made a few seamless patterns and used them for wrapping paper. I ordered them to make sure they are perfect before making them visible/available on my store. I changed the design layout to the tile option and added my patterns that way. 

When I got them, they all have a tiny white seam showing going horizontally. I know that these patterns are correct because I have tested them. 

In all of the preview photos they look perfect, they are hairline seams only visible in person. Some of them are half-drop so I have the half-drop tiling option selected. Some of them are full drop but I had none selected. Is this a printing issue? Should I change the full drop tiling option to centered or basic?

The fact that the seams are appearing ONLY going horizontally and not around each tile is stumping me. 

Thanks in advance for any insight!!

[ColsCreations]

Without being able to see the design it's impossible to guess whether it was a production issue or design issue. But, I had an issue once with a hairline seam that was almost imperceptible. Turns out one side of my background .png had a couple pixels of transparency which wasn't noticeable until tiled where it created a seam.