There will always be some people who are missed in the Census. The Census Bureau refers to these people as Hard To Count (HTC)
“Historically, the census has undercounted young children, people of color, rural residents, & low-income households at higher rates than other population groups. (See Race and Ethnicity in the 2020 Census [PDF].) Also, groups with low self-response rates in prior censuses or census tests include "linguistically isolated" households; frequent movers; foreign born residents; households below the poverty line; large (i.e. overcrowded) households; low educational attainment households; & single-parent headed households. And people who distrust government authorities and/or have been or could be targets of law enforcement or heightened surveillance may be less likely to respond to the census. In the Census 2020 HTC application, statistics on these groups for each tract are presented when a tract is selected on the map.”-What is “HTC”?
In addition, people who do not have access to a computer and reliable internet are also likely to be undercounted.
Curious about HTC areas? Check out this interactive map.