Farmer villager will eventually have his inventory filled and stop harvesting crops altogether.  This currently breaks any form of wheat farm that I can currently think of.  To "fix" this issue, in previous versions you could fill all 8 of a villager's inventory slots full of seeds and they'd harvest regardless of whether their inventory was full or not.  However, because they now compost seeds they'll simply fill up on bread, bonemeal, wheat, etc. and stop harvesting.