Thank you for posting your question on History Hub.
The 1870 Federal Population Census has been digitized by National Archives partners and is available on Ancestry.com and FamilySearch.org. These are paid subscription services but your local library may offer free access to the websites. You can also contact the National Archives at DC with additional questions about the census and to request copies or a search of the census. That location is the custodian of these records and may be better equipped to explain any omissions in the record.
We hope this information is helpful in your research.
The National Archives and Records Administration