Solution
We recommend that the model be placed into the netlist hierarchically, meaning that all child subcircuits are within the parent. For example:
.subckt parent 1 2 3
x1 1 2 3 child
.subckt child 1 2 3
.ends
.ends
If the subcircuits are not entered hierarchically, there is the potential for conflicts in the netlist, which would result in netlist errors.