DD is almost 29 weeks and can't yet sit unsupported. Is this normal? She loves tummy time (will always roll onto her tummy when you lie her down) and has had very good head control from a young age but just cannot sit for more than a few seconds before tumbling over. I do daily exercises with her as advised by HV team but to no avail. I'm very concerned now that I'm doing something wrong or failing her from reaching this milestone.