My villagers are unable to reproduce

SUMMARY

What I expected to happen was...:
I fed my villagers enough bread and vegetables to make them reproduce babies. 

What actually happened was...:
They could make new villagers like usual at first, however, after less than one hour, the villagers couldn't pick up the vegetables I threw on the ground and reproduce anymore. This issue make automatic farming impossible and I tried to create several servers to overcome it but in vain, as they all experience the same problem. In this case, can you instruct me on how to fix this annoying bug? 

Thank you!