Hi! Just wondering if anyone has experience of their LO hating tummy time? He can hold his head up when we carry him and loves pushing up when we put him on a tummy time cushion. He can hold his head really well when we carry him like an aeroplane too! But when we put him face down on a mat he just lies there and cries. He isn't pushing up with his arms at all. Is that normal? Is there any way we can strengthen his arms? Thank you!