It's always DNS.

A DNS propagation checker that shows what Google, Cloudflare, OpenDNS, Quad9 and major ISPs are serving for your domain right now, and how long each one will keep its copy. Each check also reads the registry (to catch holds like clientHold), the TLD delegation and your nameservers' own answer.

Free JSON API · How long DNS propagation takes · How to flush DNS caches · clientHold and serverHold