This question from a client this week - why is it that when I lookup from a list to a document library, I only see Word documents even though there are Excel and PDF files in the document library too?
The client was using a multi-select lookup field, but the same thing happened with a single-select field.
I recreated the scenario with a document library like this:
And in my case, the lookup returned only three choices:
It's not a content type or file type issue, it's simply that the Title field was not filled out for all the documents. When the Title field is blank, the lookup acts as if those documents don't exist.
I have often wondered why you're not allowed to look up to the Name field of a document library. To my mind, you should be able to, since Name is the primary key field for the document library, just as Title is the primary key for other kinds of SharePoint lists. But since it's not possible in this version, it's another example of why it's a best practice to fill out the Title field of documents, either on upload or after the fact.