SCOM Agent HA is not working properly

Hi Experts,

               I'm facing some issues with SCOM Agent HA which we have configured for one of our customer in different domain. We have two Gateway servers hosted in customer domain and configured as primary & failover HA using Some powershell scripts, we have also verified the O/P of HA using the below Scripts and everything looks to be fine.

$Agents | sort | foreach {
Write-Host "";
"Agent    :: " + $_.Name;
"--Primary MS  :: " + ($_.GetPrimaryManagementServer()).ComputerName;
$failoverServers = $_.getFailoverManagementServers();
foreach ($managementServer in $failoverServers) {
"--Failover MS :: " + ($managementServer.ComputerName);
 }
}
Write-Host "";

Issue: Two days back the primary Gateway server went down due to some issue, so ideally all the SCOM agent should report thro Failover server, but in my case 35 out of 60 servers are reporting as healthy rest of them Grayed out.

T/S steps: Checked the event log for Grayed out agent and found 21016 Event

Sundar