I have established basic villagers breeding farm and noticed that after certain amount of time first two villagers-farmers had stopped to breed. One of them creates hearts particles and throws bread into other, and last doesn't react any way, just continues harvesting crops (which are picked up by minecart hopper).
Assume, one of these villagers has his inventory full of wheat seeds, that's why he cannot pick up any bread, thus can't check requirement to start breeding function.