How do I troubleshoot specific Products missing from Commerce Search?
Not sure if this is proper 'etiquette' but here is the solution for -this- problem:

My Product Display has an Entity field which displays a list of 'related products' -- a common enough situation in a Shopping Cart system. The related products are a View which displays products linked to the parent product NID.

Like most sites, I imagine, the Index excludes unpublished products.

Apparently if any one of the Related Products is unPublished, this affects the Commerce and/or SOLR Search index for the -Parent- product. IOW: if any child product is unPublished the index skips the -parent- product as being 'unpublished'.

Not sure which module is at the root of this... or who to notify... but if the Commerce Guys monitor, this is a real 'gotcha'. I've run into this many times in Drupal where SQL Queries are not 'isolated' from one another... ie. a child view will affect a parent query.