What is it that you want? If you want it to have more waves then increase number of waves, if you want it to damage the units it hits over time, increase duration.
Just as I suspected... Change the Casting Time to 0.05. If you don't, it will be like the spell is not cast and I dunno why it even worked the first time.
It's because it needs a very short casting time in order for the spell to have effect. The spells seem to be madea little weird and that is why Casting Time for first execution is ignored. Spells are strange sometimes, so you must experiment and try to get how they work.