1) Which kind of registration are you referring to?
Registration of mIRC has nothing to do with registration of your nick on a network...nor vice versa
2) Registering your nick on a network will only register it on that network, not any others. Ensure that you are using the correct network.
3) If you are sure that all of the above are correct, then I can only suggest that you join the network's help channel, usually #help and see if they can assist.