I am reviving zombie villagers to get discounts, but when I try to change their professions, they wont change.

So I've been resurrecting zombie villagers to get discount books (I've been taking them from my mob dlfarm). And they used to change professions, but suddenly, it stopped working. Now the only other way I can get the books I need is fishing and enchanting tables, but they take too long and I desperately want villagers.