If you have some geo blocking you may need to update or change it.
Let's Encrypt uses Multi-Perspective Validation Improves Domain Validation Security - Let's Encrypt
Since these are Domain Validation (DV) certificates the Domain Name System (DNS) is used extensively in the validation process as well a allowing us to assist here on Let's Encrypt community.
DNS Queries need to give consistent results from any location on the Internet, all your authoritative DNS Servers for the Domain need to also give consistent results as well.