Remember that not all entries in the SSDI are created equally. My grandfather's SSDI entry only indicates a year and month of death.
Consequently if I enter his date of death-7 December 1968 in the search box, he will not be located as the "7" is not one of the fields in his entry.
Always consider making your searches less specific and keep in mind the reality of how many matches you are going to get. There won't be many Cecil Neills in the database to begin with. A death year of 1968 is most likely precise enough.