Are the hungry hearts up? If not, try getting those up first before playing with your Tama. Try different games as well, I noticed that in V4s some Tama's hearts are hard to get up than others. Try playing a few more games with it. If you get nothing still - reset your Tama. If that doesn't help, take it back and get a new one. It may be disfuctional.
Also, if you opened any letters recently containing 'poo', it'll reset your happy hearts.