Do not worry and tummy time should be started in 4th month so it is actually ok and absolutely right time do not worry she will up her head do not compare with other children's every baby is unique and if your pregnancy was absolutely normal and you did not had any problem during a pregnancy suggest high blood pressure sugar on having any placental abnormality then do not worry baby will catch up but you have to play with her not just keep her on tummy but make it a fun way play with her or the best will be to lay down her on a bed with her tummy downside and pass her a ball to play this will engage her then make her cry she will try to look at the toy with curiosity