The fifth most common cause of a BGP failure is that the network it is supposed to be advertising is DOWN (often not even connected). This is fairly uncommon because the network that BGP is usually supposed to advertise is the LAN that the Administrator actually manages, which is rarely down. Still, administrators usually refuse to connect the LAN to the router before trying to set up BGP and don't have enough expertise to know how to work around this.
BGP is doing what it was designed to do and is not advertising a route for a network it cannot reach. Customers used to make this mistake all the time. They wanted to advertise their routes, but were too afraid to connect the Internet router to the local network, thus, the Internet router thought the local network was down and wouldn't advertise the route for the LAN.