Minecraft villagers don't want to breed?

I have built a home / hotel for "my" villagers, there I have a small room that is intended for mating, I let 2 villagers in there and locked them up. I gave them food and looked through the glass (outside of the room). There were a lot of hearts but in the end there was no child but a few "annoyance clouds". I followed everything I had read: enough beds and two blocks of air up, I even gave them more than 6 loaves of bread, but these clouds keep coming. What can be the reason?

But you are very open about your crimes.
Don't worry, I won't inform the KSK and GSG9 about it.
wink smiley