How do I explain that her birth mom can’t or won’t see her?

We have an open adoption, our agency encouraged it and gave us some education…every year I send a letter and pictures to my daughter’s birthmother. We also buy her birthsisters Christmas gifts. We have always told my daughter the story of her adoption since she was a baby. The problem is, now age 7, my daughter has recently become very interested in meeting her birthsisters. We arranged a meeting but the birth family canceled at the last minute and it seems that another meeting will not be possible for a very long time as her birthsistesr live in a different city. The last meeting we almost arranged was about four years ago, and it was also canceled at the last minute. I do see the birth grandma about once a year, however. At any rate, I feel like I have made a big mistake talking about my daughter’s birthmother as much as I have. I don’t want my daughter to feel rejected…what should I do now? How do I explain to my daughter that her birthmother loves her but does not want to or cannot see her?