My jello is not getting firm in the refrigerator 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)?
It's not working because you're trying to set jello to lukewarm water - from mixing the hot and cold and then adding the jello.

The hot water needs to be added first, then the jello, then the cold water, then refrigerate, in that order.