due to villagers having a limited inventory, and when a player holds up an item they want in there main hand, if a villager has a full inventory, no matter what their trades are, they will show that item, even in bed. Give a villager enough items they will pick up. I would recommend using an employed, non-farmer for best result, experiment if you want for things such as if you hold an item they want to.
it shouldn't hold up anything, but it does