I don't think it has ever been said that it has been "years" since DG saw Serenity. She did see her regularly in the beginning after the separation/divorce. It seems that it only after CD moved unexpectedly (without informing DG) that DG lost contact, since CD also stopped taking calls from DG. We don't know for how long, or if she was able to see Serenity after that. We are told that CD made it difficult to near impossible for DG to see Serenity, despite repeated attempts. But we don't know how long it has been.
As for why DG didn't take more drastic measures, it has never been said. Perhaps she didn't want Serenity's life to be disrupted yet again, so was still trying to take care of it herself. That, however, doesn't totally explain why she (DG) didn't even know that Serenity was in the Home.
Also, I agree that when Serenity was taken out of the custody of CD, that child services (or whoever) should have at least gotten in touch with DG, since she had joint custody. That part really bothers me, but I don't know where the blame lies - with CD for not telling child services that DG had joint custody, with child services for not checking Serenity's records as to custody, for DG for not stepping forward, assuming that she even knew about the event at all. There's too much we don't know.