Baby 2.5 months old cannot hold their head up on their stomach. How can I…

Baby 2.5 months old cannot hold their head up on their stomach. How can I help? I put him/her on his back, turns the head in one direction and sucks on a fist. He/she watches with eyes at the toy, but raises his/her head for 2 seconds and lowers it. It doesn't turn to the other side...