Buffy the vampire slayer season 6 question?

ok so, i just watched season 6 and a bit of season 7. In season 6 when tara and buffy get shot, willow tries to save tara by doing that magic thing. But they dont give her the power to sace tara, but then willow just leaves tara there, then she goes to all those books and sucks the information out of them . then she goes all dark but she gets the bullett of out buffy whn she is at the hospital, SO MY QUESTION IS!! why dident willow do the same for tara? (use magic to get the bullet out of her, like she did for buffy???)