Oh no, I'm so sorry to hear this. There have been a couple of members who had this problem after one cat returned from a stay at the vet. You may need to restart the introduction if things don't improve soon. How long has it been since Carlo came back? It's interesting that although Monte is the one hissing, it was Carlo who attacked Monte. Did you happen to see what was going on between the two when Carlo attacked? Was Monte being aggressive in any way?