How do I get bio mom more into her sons life?

my stepsons mom brought us "c" last December cause she could not handle him. When we got him he was being treated for bipolar disorder. We have taken him to several doctors and finally were sent to a specialist to rule out autism spectrum disorders. They wanted all kinds of history that dad and I don't know. When I call mom to find out stuff She either skirts the questions or will call back after the appt. I needed answers for. I understand she had a difficult time with him but to get "her" son the help he needs we need her input as well. What can I do to get her to understand this?