Since you did it last year, it seems like there are three possible explanations for your present inability to book the same trip. Either (a) you were lucky last time -- somebody made a mistake and you got what you shouldn't have; (b) the rules have changed between then and now; or (c) you have lately spoken to a couple of incompetent agents.

The usual advice in situations of type (c) is to keep calling until you get someone who is able/willing to do what you want. If you call four or five more times and you still keep getting the same response, you might want to ask about the possibility of (b) having occurred.

Good luck.

cheers,

Henry