Gmail failing to send/receive sometimes using Mail

Not receiving or being able to send some gmail emails via Apple Mail. Not all gmail emails fail. Message for failed outgoing is 'connection to outgoing server smtp.gmail.com failed'. Failed incoming to iPhone (Apple Mail) are being received via Gmail App and PC apps. All emails that fail in Apple Mail, do so consistently, but not all incoming and outgoing gmail emails fail in Apple Mail.

How can I fix this?