It’s hard to say without the file. Maybe there is something white on top of this line covering it? Or maybe it’s inside of a frame which is smaller than the line and has Clip Content enabled?

Thanks @Gleb

I have exported that portion of the design into a file. The issue still appears with Line 4 and Line 5 not displaying. Hopefully, there is something pretty straightforward to fix it!

Sorry, not sure of the best method to share the file but have used WeTransfer Figma File

Thanks for your help!

In the future you can just share the file link and make sure its sharing settings are set to anyone can view. There is no need to download it, Figma is a cloud tool.

Regarding the lines, it looks like a peculiar bug. I can’t find any reason why this specific line isn’t working. It has a weird ending and when you enter vector editing mode on it, there are no vectors there. You can copy the working line there and delete this one and it seems to be ok.

Indeed - I have no idea why I forgot I could share the link. Okay, thanks for your help

This is probably the result of different screen density, a 1px line that doesn’t know where to go will not render, especially since Figma is trying so hard to adjust the artboard to your screen resolution. try placing it on a y-axis that is divisible by 8 and see if that helps, try using a rectangle with 1 px width instead of a line, the line does not align correctly to the grid even if stroke is set to center