I discovered this issue the other day.

Scenario:  Lookup field intending to bring back the Title field of a Web part page in another list.

There’s a problem.

When you create a web part page in a library, the name of the web is what you define it as, not the title.  The Lookup field type, can’t look up the name…..Only other fields like the title.

How do you get round this?

Create a SPD workflow to populate the title field, when the field is modified.