It’s always the same few reasons:
- the user was using an adblocker that blocked Matomo
- the user has disabled JS or blocked all third party scripts
- the user has enabled DoNotTrack and therefore their visit isn’t tracked by Matomo
- the user has used the OptOut iFrame to OptOut of tracking and is therefore not tracked.
- There is an error with your webserver causing some requests to be dropped (e.g. by using mod_security)
- There was an PHP error when the data should have been stored.