This is probably related with your
other issue.
Standard line width is currently 0 in LibreCAD. This is to keep line thickness independent from zoom level.
Depending on operating system and printer driver it happens, that line width 0 is printed very faint or not at all.
So try to change line width to an appropriate value, as you would select a pen with the desired thickness.
When you did not change entities line width, which is 'By Layer' default, it is enough to change line width in layer properties.
Armin
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