I think this is true in many adopted children. Respect for parents is a vitally important part of family integrity. We struggle with it as a society, but I think it is always there, to a point, even so. In the case of someone adopted out of the foster care system that respect is likely already connected to their birth parents, no matter how good or bad they may have been.
This means that it is hard for them to then transfer that same respect to a new parent, since the old one already holds the parental role. I am not sure how you deal with this, but knowing it is a respect issue may help you accomplish things you would not otherwise.
Ironically, the one child of ours who has any relationship now was the one who I proved my willingness to come back to over and over as he went through a very strong rebellious period. He repeatedly tested my resolve and I was relatively consistent throughout. I am convinced that is the reason he has any respect for me today. I proved that I would not be shaken. He has a ways to go, but at least has some connection.
The other three would seem to have no respect, having all reconnected to the birth father and mother, giving them respect to a great extent.
Be consistent no matter what. You may or may not gain this respect, but you will almost certainly not get it otherwise.