3D shader - panel shaded by roofline BEHIND panel?

I've been using the 3D shade modeler to try to place my panels o my roof. 
For the most part, it works as expected.

However, there is a single panel on the far left of my design which appears shaded when viewed from certain angles, even though in reality it should be.  The shad line appears to corresponds to the roofline of the roof that is UNDERNEATH the panel - which clearly cannot cast a shadow on an object above it.  But I've confirmed that this is more than a visual artifact - the diurnal analysis shows shade on that string when there should be none.
I've tried deleting and re-creating both the panel and the offending roof structure.  I have no hidden or disabled objects in the design.
How do I troubleshoot this?  How do I fix it?

Paul,

Thanks for the advice.  I tried it with limited success.  Sometimes it helps, sometimes it doesn't.

The point became moot later on as I moved my arrays around and the roof in question became unimportant.  However, I had similar problems elsewhere.  Strangely, the solution I found was to change an unrelated parameter of the roof - for example, the length, by a small amount.  Somehow this would help.  I still can't explain it and I expect it will happen again.

-Ron