Is she stubborn or does she not get it?

For the last couple of days I've really been working with Amelia on the two signs we've taught her: "More" and "All Done." She did them all the time for a while. I want her to be more consistent with them, so I start by giving her a piece of banana (her favorite food). After she eats it, I say, "Do you want more?" and I show her the sign. Then I help her make the sign and I say, "Oh, you want more. Here you go." and she eats the banana. Then I say, "Do you want more?" Sometimes Micah will give me his banana and I'll do the same with him when she's watching (he loves this part because he gets to be the teacher). I think she's just really stubborn. This morning I sat there for probably 20 minutes and she wouldn't say it. I kept showing her the sign and making her do it and then saying, "now you do it" but she wouldn't. Micah totally had this down by nine months and Amelia is 10 and a half months. Any suggestions?