I know some of the City Walk restaurants will only take reservations 30 days out. Not sure about Mama's.
When I received my e-mail confirmation of my PBH reservation, an e-mail address for the concierge was included. I e-mailed and asked them to work on several dining reservations for me for my upcoming trip. They did get me a reservation at Mama's, and at that time my trip was about 45 days out.