I have to admit, I am pretty bad at enforcing tummy time. She does a lot on my knees and maybe a few minutes a day on the floor tummy time. She hasn't rolled over or even tried. She lifts her head well and is starting to use her arms to prop up. Have I just not given her enough opportunity on the floor to think of rolling? Is this very late not to roll? Her back and neck are strong and she can keep her head level or even forward when I pull her up from laying on her back.