It takes some time to update the nameservers of a domain when you change them. DNS Propagation is the term for this period. You may notice some unusual behaviour on your domain/website during propagation, such as one person seeing the new server while another sees the old one, working while not working, and so on. These unusual behaviors, on the other hand, are to be expected during DNS propagation.

Usually, DNS Propagation takes up to 24 hours. But depending on various factors, it may take a maximum of 72 hours.  So if you are getting these types of issues within this time, there is nothing to worry actually.  But everything must be fine within this period. You can check your DNS Status anytime using DNS Checker. If you are facing any issue after 72 hours, then there is something wrong. So you should contact your hosting service provider.

In short, DNS propagation depends upon how frequently the DNS master cache nameservers are refreshed.