Are they trying at all, maybe just forgetting a lot, or just flat out not accepting it and refusing?
My mom had a hard time, and I did understand that, those kind of changes aren't easy to adapt to. I came out of her body as a boy, she nurtured me as a boy, raised me as such, all the while seeing me seemingly grow up into a man, all the while 20 something years calling me by the name I was given when I was born to her, Michael.
There were times during my early transition that it used to upset me really bad, I would constantly correct her (as well as my siblings) that I was a she, not a he, that my name is Megan, not Michael. I didn't want to be pushy about it, nor cause upset, but if I didn't correct her then being so used to me as I was in the past, she'd have gotten too comfortable with it and kept on referring me as him, we'd get nowhere and our relationship would have collapsed. At home it just bugged me, outside and about with people around it made things embarrassing. Sometimes she'd snip at me, that she was trying, its just not an easy thing to get used to, it takes time. I was putting her through a lot, this whole wanting to be a girl thing, some serious mental stress, and she'd constantly remind me of this. While I was living it, she had to as well because she loved me and was there with me all the time. I tried to give her a break sometimes, see if she'd correct herself, see her own slip up, but it was really hard.
But sometimes people, especially loved ones, they don't really want the change, its scary to them, like they are losing you (as well as what are people going to think?) so even though they do try, at the same time they also try to see if they can somehow keep it from being, that maybe you'll change your mind about the whole thing if they keep reminding you of who you were to them. Its not easy to let go of what you love. But they aren't letting go unless they give up on you. Over time my mom took to me like I had been her daughter all along, only once in a great while I might here a small slip up, but let it pass, because just as I have come a long way, so has she.