A common reason for this issue is down to authentication between the gateway and the Web dashboard. The following error in the gateway log can show an insight into the cause:
This is a failed login attempt to the Web Server and the Gateway is unable to connect therefore no data will be transmitted from the Gateway to the Web Server.
Therefore you'll need to check the authentication of the affiliated gateway in the Gateway Setup Editor (GSE) within the Active Console (AC).
The below screenshot highlights where these amendments can be made: