Why sometime we can see thread line in the drawing and sometime not?
are tangent edges visible?
Make sure the thread is not suppressed in the 3D model.
In 3D, View > All Annotations. May need CTRL-Q for rebuild and graphics update.
In 2D, View > Hide / Show Annotations > select thread line.
I have been seeing this lately as well and would like to know whay it is happening.
If your drawing is of an assembly, the threads never show until you insert them. Go to Insert, Model items. Change Source to Entire Model, check mark "Import items into all views", Click off "Marked for Drawing" under Dimensions and click on "Cosmetic Thread" under Annotations. I usually wait until I have all the views on the drawing I want. Sometimes you then have to delete off the notes for the thread size for every tapped holes in the drawing.
Retrieving data ...