Have you tried using Heading mode temporarily instead of LNAV? When nearly at the next waypoint switch back to LNAV, to see if it will then follow the route. If it still won't then on the LEGS page of FMC replace the waypoint it won't follow with the one after that. When replacing the top waypoint it is a bit different than a lot of payware aircraft compared to the default. I cannot remember exactly the name of the key to press but this key has to be pressed first to enter the next waypoint on the route. Will it follow after that?