Content blockers ship filter lists that block requests to known analytics domains. When a visitor with one installed loads your site, the request to the tracker is cancelled before it's sent - so that visit never reaches OakData, and your numbers quietly undercount.
Confirming it's the blocker
Open your site in a private window with extensions disabled, or on a device with no blocker, and watch Live. If the visit appears there but not from your everyday browser, a content blocker is the cause. In the Network tab, a blocked request shows up as (blocked:other) or simply never fires.
The fix: serve OakData first-party
Point a subdomain of your own site (like analytics.yoursite.com) at OakData with a single CNAME record, then load the tracker from there. Now the request is first-party - same site as your pages - so filter lists have nothing to match, and you recover the visits blockers were dropping.
Managed reverse proxy
OakData can host this for you - one CNAME and you're done, no server to run. The full walkthrough is in the reverse proxy guide.