Minecraft villager breeding?

I brought 2 villagers, a farmer and a librarian, to a new (empty) village to restart it, but they won't breed. There are enough doors for 2.5 more villagers, i have traded with both of the villagers 3-4 times, sometimes at the same time, and i've even locked them in a 4-3 area together for a cupl minecraft days. Its been ~10 minecraft days and they don't do anything besides 'socialize'. What's the problem?