When I started a villager profession farm, I would place the necessary items for the villagers to change. Not to trade with them and move them to a secure location. No other villagers in a 100 block radius. When I place the lectern in this example. The green lights show up that it has changed profession, but not the villager I’m trying to changer.

PS. Villager has the a bed and a house also this is the 1.16 beta