Suddenly, color 41 start showing up (while plotting) very shy almost dotted line... How to control line weight while plotting...?

Hello jaalmanzar, it looks like you have not set your print correctly.
You must set the .ctb to at least monochrome or with an other of your company.

Lineweight can be controlled within the drawing itself or via your .ctb or stb.It would be nice to have more information or better yet a copy of the DWG file that we can look at ourselves.