Is the scale defined at Windows 8 level set up to 100% ?
Since Windows 8 was released, there were several issue reported here about scaling interface at Windows level.
This is also an issue in Windows 7.
does shifting up/down the line for 1 pixel make any difference?
is the line inside of a layout element or on the border? Do the positioning of object (line) with mouse OR arrow-key's first (not by inspector) in order to have object assigned to the right layout-element.
Change to browse mode and back to layout to set correct position with inspector. FMP will recognize that object as part of that layout element, even when object is on the border to next layout element