The propagation of DNS is a progressive and non-linear phenomenon very difficult to predict and to monitor.
There are online tools that allow us – in broad terms – to do so, comparing the data reported on some DNS servers around the world and comparing them with those of the “root server”, or DNS authoritative servers.
The main tools that can be used for this purpose are:
Not all DNS servers update as they should!
Unfortunately, individual DNS servers are left to the discretion of their provider.
It, therefore, happens that, in some cases, they are poorly configured, and therefore do not update with the due frequency.
For example, some providers update some of their DNS servers only once a week;
so users who use those DNS servers of this provider “will discover” the new configuration only after a few days … (up to seven, depending on whether you were lucky enough to do so on the eve of the update or the day after …)