The OCLC Link Resolver is designed to determine the genre of an item based on the metadata being pulled from a vendor's database. If the genre artifact in the vendor's database is "dissertation" or "dissertation & article" then the link resolver will designate it as such and cause full text links to not surface.
You can see what genre artifact the OCLC Link Resolver pulled in by looking at the "genre=" part of the link. For example:
testing.site/atoztitles/link?genre=dissertation