The reason it’s not working is because you are trying to add a value to an array in a structure in an array. (from what I can see) When your doing this, always assume your getting a copy and never editing the original. What you need to do is save the array of the sub inventory slot as a local variable, add the new entry and then use that to rebuild the sub inventory array to set the array element in your main inventory array.
I hope this make sense. 