How can I make my farmer villager replant crops? (1.8)

Throw them two more stacks and try again.

According to the "Villager" page on the wiki they will pick up 8 stacks before their inventory is full, not 6:

The villagers will pick up 8 stacks of items before their “inventory” is full.

After their inventories are full they will stop picking it up:

After their “inventories” have been filled up, they will continue to tend to crops, but will not pick up anything they collect.

You should also make sure that you have chosen farmer villagers to do the work. It will not go down well if you have chosen a butcher or a librarian to do the farming work. Make sure you farmer villagers they look like this:

Also, in 1.9 and above, right clicking (trading) with the villager will tell you the profession. This is important because there are other villagers with plain brown robes now.