If the agency is not willing to mediate or you went through a lawyer, give me a call and I will try to find a mediator in your area. Now it has been going on 3 years since the adoption, and the cards and pictures from the adopted parents have slowed, and we did not get a reply from our last letter. We only sent a letter every couple of months or so through the agency. The most likely reason is that things are just getting more hectic in their lives. It is easy for people to put off things like writing letters, etc.
Parents often take less pictures as the child gets older as well.
A more serious reason is that they are losing interest in working on the relationship. There could be a number of reasons for this. In semi-open situations especially, it is easy for adoptive parents to forget that contact with the birthfamily will ultimately benefit the child. They may also have little or no support for ongoing corrispondance. Often, if they have no support at all, they will come to believe these things themselves.