Full hearts are different than the full stars as seen underneath your ryu on your
(you need an account to see links) page. When people stop by and baby sit your ryu, the hearts will fill up. If you desperately wanted to branch hop via death of ryu method, you'd turn off babysitting and then wait 4 or so days until it gets a grave and a halo overhead. However that is not recommended at all as deaths will limit your ability to evolve up future ryu branches, such that you might never be able to reach a dragon or cerebus for example. Ryus are meant to live and thrive and be happy! I would 100/10 recommend trying to trade before that. Your stars for you ryu also are determined by your care of others' ryus, so even if you don't care for yours but you do for others you still might reach 5 stars.
Ryu's evolve every 3-4 weeks. They take time to grow up.
Does this help?