ISO: question about freezing ravioli

I love to make ravioli from scratch. We use a cheese filling that is mostly ricotta with parmegiana, mozzarella, and egg. We freeze them on cookie sheets and then store in freezer bags. If we eat them within a week, they are terrific. But after two weeks or so, they seem to develop an "off" flavor. Does anyone have any idea what could cause this? My grandmother and her sisters used to make and freeze ravioli all the time, and it would be good 4-6 weeks later (maybe longer, but they were usually eaten before any more time could pass). Any suggestions?