Perhaps one of the biggest plot holes in the Harry Potter universe is the existence of Veritaserum, a very powerful truth potion. Though it is said to be hard to brew, the potion is underused considering how useful it would have been to characters like Sirius Black and institutions like the Ministry. Even if the potion is hard to make and not always reliable, neither the books nor movies give an explanation satisfying enough to warrant the infrequent use of Veritaserum.
Source：https://www.cbr.com/veritaserum-potion-changed-harry-potter/
