Shooter problem

[Neilski]

I have a shooter firing arrows, OK. It will fire three lots of arrows, one at a time when the party, monster or object steps on a specific square. For some reason I cannot get it to do this properly, it either fires one set of arrows and then none at all, or all three sets at once. Am I doing something really stupid? I have a trigger setting off the shooter, but whatever I try I cant get the trigger to 'reset' itself. I managed to do this years ago but somehow it wont work whatever I try this time. Any ideas?