How do you deal with communication issues about a loved one's mental health condition?

Mom's been struggling with depression for a while now, but trying to talk to her about it is like pulling teeth. She either changes the subject or insists she's fine when I know she's not. I'm worried about her, but I don't want to push too hard and make her shut down completely. I want to be supportive, but I feel like I'm walking on eggshells all the time.