figured it out (its not me, its the template - sorry). there is some tricky logic going on here:
1) for related posts to work, there is some connection between the tag DISPLAY name and its SLUG name
2) for tag's whose DISPLAY name is one word - the SLUG is auto generated as that one word and everything works swimmingly
3) for tag's whose DISPLAY name are two words - the SLUG is auto generated as the "two-words" and NO RELATED POSTS RENDER
4) the only way to get related posts to render for tag's whose DISPLAY name are two words, is to modify the SLUG to ONLY be the FIRST word. So, for example:
tag name: This Tag
tag slug: this-tag
will not render related posts even if there a multiple posts tagged with "This Tag"
if you modify ONLY the tag slug from: "this-tag" TO "this"
All posts tagged with the "This Tag" will show the related posts beneath the post.
very bizarre logic. Can't we just tie it to the slug name or even easier slug_id?
while you all be providing a fix for this? while you're at it could you describe briefly the best method to change the "?" image for related posts that don't have featured images? I understand the functionality can't be undone by you, but a point to where we could definitively change the default image for that would be helpful.