AutoSSL with several domains

I'vee got several AutoSSL errors in my server. Gone through earlier posts, mostly seeing "This can happen if your server's firewall is blocking access attempts from Comodo to validate the certificate". I have several domains running on AutoSSL (thanks to Cpanel making it easy) but some of them are having problems; i.e. it's not a firewall issue. Tried to disable and enabling AutoSSL for a domain but did not help. Any ideas? Thank you in advance.