I have heard of passive and active DCC but not this.

Well... it's the same thing.

An active bot will initiate the connection, a passive will wait for you to connect. If you are under NAT then the active bot won't be able to connect to start the transfer and the transfer will just die without you knowing.

The passive bots are identified with a 'p' in the name. Usually between pipes: |p|

Unfortunately more and more channels don't indicate if the bots are passive or active. This fucks up the user experience, specially if you are in queue for hours and then the transfer fails. I stopped using xdcc because of this. There are still some good channels though...

Try to use a passive bot and see if it helps.