This last time I made Antonia74's RI it came out rock hard. I did add glycerine and lemon juice 1tsp each. The other two times I made it per the recipe and it came out firm outside but soft inside...any reasons why it came out rock hard the last time? I though glycerine made it soft. So is lemon juice my issue? I thought it helped taste and increased whiteness of icing.

Humidity could play a factor....

I'm not sure, but I thought people added 1 tablespoon of glycerin/corn syrup to the recipe....not just a teaspoon?