If that doesn’t identify the problem, ask for help in our Community Forums. Test your site with SSL Labs' Server Test.
If you’re having an issue with modern platforms, the most common cause is failure to provide the correct certificate chain. If your certificate validates on some of the “Known Compatible” platforms but not others, the problem may be a web server misconfiguration. After September 2021, only those platforms that trust ISRG Root X1 will continue to validate Let’s Encrypt certificates ( with the exception of Android). Some platforms can validate our certificates even though they don’t include ISRG Root X1, because they trust IdenTrust’s “DST Root CA X3” certificate.
The main determining factor for whether a platform can validate Let’s Encrypt certificates is whether that platform trusts ISRG’s “ISRG Root X1” certificate.
