laying two or more horses in a race

any help on this would be appreciated. i have a few systems which i use and i do quite well but i just can't make it pay laying more than one horse in a race. i don't lose massive amounts laying more than one in a race-about 80 points in the last 2 years. thing is because i'm not a bookie, who does lay all the runners, i just wondered if this was bad luck or if the odds are against you laying multiple horses. i could dutch if i get more than 1 selection but statisticaly this shouldn't work-should it? its actually worse on the all weather than on turf. i only lay odds under 11.