harry potter - Why did Bellatrix rely on torture and the word of a goblin to get information from Hermione, and not use the Imperius curse?

Clearly Bellatrix liked torturing, but given her awareness of the importance of finding where Hermione got the sword in Deathly Hallows, and that they were "close to death" if her vault had been entered, why didn't she use the Imperius curse to get an answer when she really needed it?

She had no reason to think that Hermione knew how to resist it, which was rare anyway, and instead switched to simply asking Griphook whether the sword was genuine, surely not as reliable?

Update:

From "Goblet of Fire", duel with Voldemort:

"A little break," said Voldemort, the slit-like nostrils dilating with excitement, "a little pause... That hurt, didn't it Harry? You don't want me to do that again, do you?"

Harry didn't answer. He was going to die like Cedric, those pitiless red eyes were telling him so... he was going to die, and there was nothing he could do about it... but he wasn't going to play along. He wasn't going to obey Voldemort... he wasn't going to beg...

"I asked you whether you want me to do that again," said Voldemort softly.

"Answer me! Imperio"

And Harry felt, for the third time in his life, the sensation that his mind had been wiped of all thought... Ah, it was bliss, not to think, it was as though he were floating, dreaming... just answer no... say no... just answer no...

and again, from same book, Barty Crouch's confession:

Then I packed up Moody's clothes and Dark detectors, put them in the trunk with Moody, and set off for Hogwarts. I kept him alive, under the Imperius Curse. I wanted to be able to question him. To find out about his past, learn his habits, so that I could fool even Dumbledore.

Harry isn't prompted or told what to say, in effect he is asked to reply with information (what he wants), by means of the Imperius curse. Although we can't be sure, it sounds like Crouch is describing himself using the Imperius curse to compelled detailed information too (he didn't need the Imperious curse just to keep him prisoner, and he explains that his motive is to get detailed information that Moody ordinarily would never have given, but doesn't imply any other means was used to get it).

So both Voldemort and one of his highest lieutenants resort to Imperio first of all, to get information and answers (including from a strong minded Auror), making it likely Bellatrix (a prodigiously strong witch) would have seen/known the same.... making it even more unexpected that when she knew her life was at risk and she needed to know the truth, she didn't use it too?