The procedure I followed this time:
I started using the microwave to heat up the water. Then I pour both cold and hot water in a bowl, then the package of jello.
Previously I always cooked on the stove. After boil I would pour the jello in the pan, then to a bowl and then the cup of cold water. Never had a problem with that.
With the new method my jello is not getting firm in the refrigerator. Why (not)?