RDS 2012 UnifiedSessionID

We have had RDS 2012 in production for about a month or two. Every once in a while i am getting a user with a empty UnifiedSessionID. Any thoughts on what is causing this? 

Get-RDUserSession | Where-Object {$_.UnifiedSessionID -eq $null} | fl

When this happens the user seems to be hung. Cant login to the environment. I cant kick them out with the  

Get-RDUserSession | Where-Object {$_.UnifiedSessionID -eq $null} | Invoke-RDUserLogoff -Force

as it says access denied.

I am not even really sure how i am fixing them... after some tinkering they are fixed. This morning i rebooted the VDI management box and it cleared it up.