Refusing to Sit

For whatever reason, Tali does not want to sit. Almost every time that I try to get her to sit, she stands. If she does actually sit, it's in the tripod pose and only for a few seconds. I'm not sure if it's because her back muscles aren't strong enough or she just simply prefers to stand, but it's so frustrating! I'm not worried (yet) that she's not sitting, but I just wish she would. She's the only one in her music class that isn't sitting on her own. I see how the sitting babies play and know she would have more fun if she sat. The only way I can get her to sit is on my lap or in the Bumbo but that's probably not helping strengthen the right muscles. Oh Tali, why won't you sit?