Hard to say for certain with such a brief description... but Im guessing you havnt got a feed to the switch for the outside light. If the incoming 'feed' is fed to the hall way switch terminals then there is no 'link' to the second gang switch (outside light). Take a little bit of the brown core and place a link from the permanant live (of hall swich) to the common (of the outside) Good luck...
Bright spark, August 2008