At any given time of day an unemployed non-nitwit villager will not accept their job, I have tried composters and lecterns and yet they will still not accept it. I tried walking away and coming back, I tried breeding them and see if the baby villagers (now adults) will accept it and they don't. I made sure that the workstations were accessible to the villagers. I have done this before and it works so I am pretty sure this is a bug.