Is your dog floppy?
I have 2 male dogs. A German Shepherd and a Coon Hound.
My Coon Hound is floppy, when we play, he'll let me flop him around, on his back, turn him around and he's relaxed. I've seen other dogs who just flop down and don't resist when you roll them over.
My German Shepherd on the other hand, will not budge. I can't roll him over. He's always on guard or ready for play. But not flopping around. When he's laying down, sometimes I'll crawl up beside him and he immediately sits or stands up. Maybe that's a Shepherd thing.
My friend has a Siberian Husky who will roll on his back for you to rub his tummy. He's a big floppy dog, but not my Shepherd.