How do Villager change jobs?

I have a couple of Villagers locked up exactly 5 in a cellar with 5 beds and would like to trade with them. One of the 5 is a librarian, the others have no job. Whenever I set up a lectern, however, no one else takes on a job. I have already traded with the Librarian, which is why he never changes profession. Maybe you have ideas on what that could be?

Minecraft: Java

Version: 1.16.1

play on a server

Do you also set up 5 lecterns?