Tummy Time Frustration

So my daughter is 6 months old. She manages to flip from back to tummy without too much effort, although the arm she rolls onto always seems to get stuck under her. She can't seem to grasp the concept of using her hands and arms to push up while she's on her tummy. She can sit up pretty well, yet not completely unsupported and definitely can't get into a sitting position on her own. On the rare occasion she has also gone from tummy to back. Is anyone else having this problem? She just doesn't seem like she's figured out upper body strength yet.