"Unit" Has not checked in for more than 7 days

We have around 110 units in a factory/office area with multiple Vlans and Group assigns.
50% of them have stopped responding to the server. The only way to get them respond again is factory reset the unit.
If you open WMS Manager on the unit it comes with error "Cant connect to server, Please try again later"

It only runs a RDP witch works perfectly. Internet works.

The Dell Wyse Management server is running the following version.

The clients varies in models and OS versions. Not all OS Versions in picture are in use.

 

We have tried to update units. Not working always.
We have tried to change VLAN assignment but they have same problem.
One unit can be fine and other one just beside it have the problem. The same model, same OS version, same Network/Switch. 

They can work for 1 week or 10 weeks and then it happens.

 

Any one have any idea on what causes it ?