Unable to activate Monitoring & Asset Management

Hi,

I have some problem activating the Monitoring & Asset Management...

To easily manage the servers of our customers we created a group and every technicians is free to install team viewer in a new machine and add it to that group.

For several machine if I trt to activate the service I receive this error (it is in Italian, I try a translation):

"Account assignment failed. The device is already assigned to another user". 

I cheched on a server for which I was able to activate the server and a server for which I wasn't able to activate the server. The assigned to user is the same...

As an additional clue on the web side of TV in the "computer & contacts" section there are several machine for which the service icon is missing.

Is there something I can do?
Many many thanks,

Eljenne