Unable to query client computers under SBS 2008 Console

Under the Network tab of SBS2008 console, I receive a message that it is unable to query for computers in the network.

This was working up until about a week ago when we had just 5 computers connected, but that is now up to 15 and it's not recognising any!  Thought it may have been a CAL problem, but we have 25 for the system at this time!

I know it's a bit vague, but appreciate any pointers to help sort it out, or do I just ignore it and delete the entry from the monitoring list?
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