It's probably because the host cache used to bootstrap is now too old. You may want to Google for GWebCache (or BeaconCache), then pick a Gnutella host IP and try to connect to this host manually. I remember Gnutella2 connection was failing, so I fired up Shareaza, noted a G2 hub IP and manually connected to this hub with Gnucleus: Gnucleus then updates all its data and works like a charm after that, no need to repeat that again.