I don't think you did anything wrong; I think that there is a setting you should know about. If you dig a little deeper, you'll find an input (it's part of an input cluster) called hysteresis. If your trigger occurs, the hysteresis determines when the trigger can happen again. For example, if your trigger is set to rising edge at 5 volts and your hysteresis is .1, then when your trigger occurs at .5, the voltage must go below .4 in order for the trigger to occur again.
Try that first. Hope that helps.
Mark