What did you do?
Did you use ice cold water? Did you use a hand blender and whizz it up for at least a minute (much longer than you think you need to). Did you add the mousse mix to the packet and mix it up before adding it to the water? You need to add it to the water,
not put it in the beaker and then add water.
Did you put it in the fridge for five minutes to set?