Tummy time

So, I feel so behind.. I just started my daughter with scheduled 3x a day tummy time on the actual play mat. I have held her on my chest and she raises her head. It’s not too late is it? I wanted to start her ASAP but she had some stomach issues that her doctor and I are finally getting under control. She only raises her head if she’s on her boppy, not completely flat unless on my chest. Advice to help improve tummy time?