hmm..I wish I knew how to help. Royal icing should dry hard regardless of how thin or thick the icing is.
Are you in a very humid area or house? Warm weather and humidity can really affect how long it takes RI to dry.
POssibly trying letting it dry in a cool place in the house (if it isn't already-but not in the fridge) or with a fan blowing on it.
Also, I'm sure you know this, but just in case-it needs to air dry sitting out, not in any sort of airtight container.
Other than those suggestions, I really have no idea what else to tell you! Hope you figure it out!