Are you challenging your friends directly from the Friends section, or are you playing against members of your squad/group? In order to qualify for the daily mission, you must directly challenge a friend via the Friends section, as that is the way to play a head-to-head duel. Anything else will be considered a group/community duel and not count towards your tasks.

The most common reason people do not get credit for finishing this assignment is because this daily assignment only considers friend-to-friend duels and not duels via squads as qualifying for the mission. When it says head-to-head it literally means a dual started against one friend only. If you are winning squad duels then this may be the reason for the mission not completing on your account.