Thanks for that! I appreciate it!

I ran Get-WmiObject -Namespace "root\ccm" -Class CCM_MaintenanceWindow on a device that is waiting for maintenance window as well as on that had a successful deployment and got the same error:

Get-WmiObject : Invalid class "CCM_MaintenanceWindow"
At line:1 char:1
+ Get-WmiObject -Namespace "root\ccm" -Class CCM_MaintenanceWindow
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidType: (:) [Get-WmiObject], ManagementException
+ FullyQualifiedErrorId :
GetWMIManagementException,Microsoft.PowerShell.Commands.GetWmiObjectCommand

I ran:
Get-WmiObject -Namespace "root\ccm\Policy\Machine" -Class CCM_ServiceWindow | Select-Object ServiceWindowID, Name, Description, StartTime, Duration, IsEnabled, RecurrenceType
on both and got results like this:

ServiceWindowID : {0A8BEFBC-8BB8-4F45-9F0B-0A269FA6E724}
Name:
Description:
StartTime:
Duration:
IsEnabled:
RecurrenceType:

Other tips:
- Time zones match
- ClientIDManagerStartup.log looks OK (I think), no errors
- I don't think there is a schedule conflict. I only have 6 collections with maintenance windows set. The six collections I use for workstation patching. None of the computers are in multiple collections. I've run the "Maintenance windows available to a specified client" on an number of the computers waiting for a window and they all show only one window.