WinRM - RPC Server Unavailable - can telnet and services are running - powershell

Question

Problem summary:
I'm attempting to check for WMF4 on all servers in a particular OU in our domain. However, I'm getting RPC errors for most servers, despite having GPOs in place to enable WinRM to work. I am running the below code from the Domain Controller.
Things I've checked or tried:
Telnet from DC to target server on port 135 and 5985. Works fine, so
firewall(s) should not be an issue.
Checked that the following services are running on the target server:
WinRM
Remote Registry
Remote Procedure Call
Set WinRM trusted hosts on target server to "*" (just as a test)
One item of interest - There is one server where I don't get an RPC error and it's the only one in the OU that's in the same /24 subnet as the DC. Seems like a curious coincidence.
What have I missed?
Thanks.
Here is my Powershell code:
Import-Module ActiveDirectory
$searchOU = "<removed>"
$2008servers=(Get-ADComputer -Filter {OperatingSystemVersion -like "6.0*" -or OperatingSystemVersion -like "6.1*"} -SearchBase $searchOU)
$2012servers=(Get-ADComputer -Filter {OperatingSystemVersion -like "6.2*"} -SearchBase $searchOU)
ForEach ($2008server in $2008servers) {
$server = $2008server.Name
try {
if (Get-HotFix -id KB2819745 -ComputerName $server -ErrorAction Stop) {
Add-Content "WMF4 is present on $server" -Path "C:\Users\`$peterb\Desktop\WMF4-Present.log"
}
}
catch {
if ($Error[0].Exception.Message.Contains("The RPC server is unavailable")) {
$ErrorMessage = $_.Exception.Message
Add-Content "$ErrorMessage on $server" -Path "C:\Users\`$peterb\Desktop\RPCUnavailable.log"
} else {
if ($Error[0].Exception.Message.Contains("Cannot find the requested hotfix")) {
Add-Content "WMF4 is missing on $server" -Path "C:\Users\`$peterb\Desktop\Missing-WMF4.log"
} else {
Add-Content "$ErrorMessage on $server" -Path "C:\Users\`$peterb\Desktop\OtherErrors.log"
}
}
}
}
ForEach ($2012server in $2012servers) {
$server = $2012server.Name
try {
if (Get-HotFix -id KB2799888 -ComputerName $server -ErrorAction Stop) {
Add-Content "WMF4 is present on $server" -Path "C:\Users\`$peterb\Desktop\WMF4-Present.log"
}
}
catch{
if ($Error[0].Exception.Message.Contains("The RPC server is unavailable")) {
$ErrorMessage = $_.Exception.Message
Add-Content "$ErrorMessage on $server" -Path "C:\Users\`$peterb\Desktop\RPCUnavailable.log"
} else {
Add-Content "WMF4 is missing on $server" -Path "C:\Users\`$peterb\Desktop\Missing-WMF4.log"
}
}
}

Answer 1

I usually get RPC errors in PS when WMI is either blocked by the firewall or when the service is not running.
If I'm not mistaking get-hotfix uses WMI instead of WinRM to remote to a machine.
Starting the WMI service and creating the firewall rules to allow WMI should solve the RPC errors.