SOME clients can't see the server, or ALL clients can't see the server? If the answer is some, then you might have the same issue I had which is duplicate UUID among 2 or more clients, which only allows the server to see one of them. Here's info from Nigel Bree on how to determine if you have this issue:
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The UUID isn't easily visible in the console UI, but you can look in the "ngserver.log" file on the console machine (that's what I use to diagnose this problem. Lines like this show incoming information from the clients:
Got status Message<Status>{ ProductVersion = #"110.00.1508", Name = (#[0x00 0x15 0xC5 0x3A 0x7E 0x52] #[0x00 0x16 0xCE 0x74 0x03 0xB5] #[0x00 0x16 0x41 0x76 0x65 0xA4]), Version = 720896, Status = Idle, IPADDRESS = 3232271617U, Sequence = 137589804, Uuid = #[0x44 0x45 0x4C 0x4C 0x51 0x00 0x10 0x4C 0x80 0x37 0xCA 0xC0 0x4F 0x4C 0x31 0x53], Build = 9999, Platform = Win2k, SUBNETMASK = 4294967040U }
You should be able to spot the UUID value in there; if you search the log by the MAC addresses of the two machines, you can then extract the UUID values they are sending and see if there is a duplicate.
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If it is this problem, Nigel can point you to the fix.