Why did 4 days pass after the NS change for the domain, and some users end up on the old site?

Question

Domain Name System

The domain has been registered with a registrar. Then it was sent to third-party NS.
Recently changed NS to others. It's been 4 days, but 60 out of 90 users per day end up on the old site.
How can this be, what can it be connected with, what can be done?

Answer 1

Most likely, the change of NS did not pass for the providers of users who visit the old site.