This could be related to current communication issues in your cluster of Web Gateways. Please check the current time on all attached appliances and also add a local NTP server on all appliances/installations. If this will not solve the issue I would ask you to check the errors in the mwg-coordinator.errors.log on both appliances:
GUI -> Troubleshooting -> Log Files -> mwg-errors -> mwg-coordinator.errors.log
It should point you directly to the root cause of the communication issue.