Missing Lines

In the attached picture you can see a half-circle but actually, it is a full circle. A few segments of the line are not visible. Could you please tell me how to solve the problem?

You could start by posting the file (just the offending geometry) and also run the Rhino command SystemInfo and paste the result here - that way there’s a chance someone can help you. Could be a video driver issue, but I’m just guessing.
HTH, Jakob