Pratchet's witches (and wizards) are vastly powerful, but almost never use their power. Wizards because they can't their heads out of...Erm...
The witches don't because it's just plain wasteful and wouldn't work anyway (Sound and fury signifying nothing. Typically the more powerful a thing is on the Diskworld, the less useful it is. Unless it's the bad guy. Or the Luggage.). Witches, in general, deal in practical knowledge and psychology to get what they need done.
Because of the latter, witches tend to get something of a pass when weird stuff happens around them. Because, witch. If people are concerned about a thing, and witch is involved, it's best to let her deal with it. Because she's not going to give you a straight answer most likely. Or she'll give you such a straight answer your mind will be twisted in knots. And then you'll have to join the quest, and probably still won't understand what's going on.