my daughter is 17 months old and has been standing with support since she was 11 months old. she started walking holding hands at 13 months. she is very active. she can climb up and down the beds and sofas on her own. the problem is she still doesn't stand on her own. when we try to leave her hand she seems to have the balance but as soon as she realises no one is holding her she sits down. what should I do?