Some messages won't download from Zoho mail account via POP

Hi, need help troubleshooting a POP problem. New test account. Mail is flowing into the account OK and messages are visible in the web interface, but not all messages (particularly recent) are being pulled off in POP access to the Zoho account. I get the same problem on desktop (Apple Mail), iOS and Android clients. Apple Mail "Account info" utility shows the messages there in the account but won't pull them off. a