For some reason, when I download a county-level table on 2017 sex, age, and education, I only end up with 838 records when I’m expecting ~3100 counties. Do you know why this is?
Downloading my data from: NHGIS Data Finder
It sounds like you requested 2017 ACS 1-Year Data. The 1-Year Data identify only areas with a population of 65,000 or more. Most counties have fewer than 65,000 residents. To get data for all counties, you could request data from an ACS 5-Year dataset, which identifies all areas regardless of population.