Baby doesn't move unless I'm on my back

If I lay in my sides, my baby will sometimes kick my ribs. But if I lay on my back she actually goes crazy. My doctor told me to count kicks every night to make sure I feel 10 in one hour, but to lay on my left side to feel them. However I only really feel them when I'm on my back. So idk what to do just count laying on my back? I'm 28 weeks