try harder and don't give up until you get it !
i'm an instructor at an outdoor camp and last week i gave slacklining initiations of 1 hour to groups of 15-20 kids and on average i had 2-5 kids walking the entire line within the first hour (without prior experience)
there isn't really a trick to it besides trying until you get it ...
just bend your knees a little, place your feet in the running direction, one after another, look at the end of the line or a fixed point at the horizon and maybe ask one of your buddies to hold you up at the back of your shirt for some extra balance until you get the 'feel' ...
oh and also last week i had a group from a sportschool, every single one of the kids in that group did a backflip off the line, most without even being able to walk it, dat shit cray!