Not everything is online. It takes years of work to type or scan and then index records. Digitization of slave records in North Carolina began about 2014, and more counties' records are gradually becoming available.
Try different spellings. Ancestry.com will find close spellings, but HeritageQuest will only search for the exact letters. Some of these county sites require actual spellings, such as Philis instead of Phyllis. You can try searching with the first few letters of a name.
If you know an ancestor's surname, try searching for that name as the owner.
From the Hege Library main page
Select the "Databases" tab
Next to "Select a Subject," select History in the drop-down menu, then click "Go."
African-American Newspapers: the 19th century
American Slavery: A Composite Autobiography (Slave narratives listed by state, then name)
Digital Library on American Slavery (Search state and county records by name. Browse subjects for a wealth of information about slave life.)