When I try to connect and if my nick is already used, mirc (6.16) doesn't try alternative nick. Then it disconnects and tries the next server and of course it doesn't work (because it's the same network). Perhaps it can help, I'm on EFnet
do //echo $me and //echo $anick when offline if same nickname is echoed then you know whatz the problem so change your anick with /anick your-alter-nick
Why go through all that trouble (typing //echo $me, etc) when it would be easier to just press ALT+O and see what's entered into the nickname and alternative editboxes?