There can be a number of reasons for this issue: you may be using a nickname that someone else has registered; the server may require you to send something like "/quote pass" followed by a number or word; the server may not be processing the CAP request (affects older servers that have not been updated); or the server may be experiencing a temporary issue. You can type the command "/debug @debug" in the status window in mIRC which will open a debug window. Now try connecting again and the debug window will display exactly what mIRC is sending/receiving to/from the server. This may help you track down the issue.
