I have an interesting problem where all my trees, Mega and mini, do not show up on a preview even though they are marked as Al previews.
If you look at the all models preview in the atached files you will not see the various trees nor te sleigh and raindeer models on the preview, but if you switch to All, you will see them. If I add my floodlights or verticals to all previews, the tress and such disappear.