Missing object while render, only background remains?!

I have a very annoying issue while rendering my drawings in AutoCAD 2010. Everything is just fine, modeling the objects, applying materials to them, even playing a little with their settings, applying different background colors and lights, etc... I render my drawing and it looks just fine. Could be better, I know, but it takes more time to playing with many settings... and then suddenly, this annoying problem come up. When I try to render it one more time, after changing some settings in hope to see a batter result, there is NO OBJECT any more in rendered image, only the gradient background remains there, but the object has just disappeared from the photo, like it was never there. After hours and hours of trying I couldn't fix that problem in my drawing, so I just copy and paste whole object to a new document and render it allover again with no problem. Of course, all previous rendering settings are gone...

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