Dear Tom,
I've had similar problems before, and although I find this kind of sun effect
can be tricky depending on the area you need to illuminate, I think I know
where the areas to explore are. You should look at the spotlight's bias and
map size. Also, notice if your base unit is a foot. This can often require
you to lower the map bias substantially.
Basically you probably have to lower your map bias and increase your map size
to get the shadows to work right. I hope it helps.
Matthew Harrison