Buswa :
Whats wrong with the fullback in this game? He doesnt tackle very well, doesnt even attempt to defend or cover off a runaway try. Its as if hes just a player on the field that does nothing apart from kick. By the time he thinks of even attempting to stop a player its too late.
Could partly be due to your general strategy and individual settings. Also consider what the opp attacking players may be set up as.