pattern replacement in sections

[Robert21763476b47s]

I have replaced a pattern in a picture however I do not understand why it splits the pattern in 3 sections with white lines showing in between.

[Myra Ferguson]

It appears that the white is part of the pattern. However, even if you remove the pattern, the shadows will repeat and the stripes won't match.

Instead of using the pattern directly, you might try using the Remove tool (it's nested with the Spot Healing Brush) on it. If you click on the pattern, Photoshop will ask you to rasterize the pattern. Once it's rasterized, then use a larger brush setting with soft edges to remove the white, align the stripes, and smooth out the shadows.