At the end of Twilight Saga: Breaking Dawn Part I movie, Bella didn't turn into a vampire for a long time even after being bitten for multiple times. Why didn't she turn into a vampire soon as everyone else always does? What took her so long?
In fact, Bella's transformation, as explained in the second half of Breaking Dawn, goes rather quickly, all things considered; she's "unconscious" for about two days. Normally, it takes three, maybe four days. The speed of the transformation in Bella's case was likely due to the amount of venom Edward put into her, especially the injection directly into her heart.
The concern of the other vampires, both in the films and the book, is that Bella is unresponsive; still breathing, heart still beating, but not moving. This is not normal; all the other vampires stayed awake, conscious and screaming the whole time. Bella's experience is a result first of the morphine preventing her from moving even if she wanted to, and second of Bella's focus, intent on not causing Edward any pain at seeing her suffer once the morphine burned off.
In the films, showing the full scope of the transformation of vampires would be dead air; three days of screaming, writhing pain. You get the idea in a few seconds that it hurts to be turned, worse than anything you could imagine, and this fact is mentioned explicitly back in the first film. Even in the books, only a few major characters' transformations are even mentioned; Edward mentions the pain of both his own and Carlisle's transformations, as does Rosalie in hers. Jasper tastefully skips that part of his backstory, and if Alice mentions any pain in her abbreviated backstory (she remembered nothing of the vampire who turned her or even of being human, so there's not much to tell) it gets maybe a sentence.
Rosalie's transformation was detailed more than anybody else's besides Bella's as part of her backstory in Eclipse, and even then it was maybe a page of the book. Bella's transformation gets an entire chapter in the second half of BD, and even that was glossed over in the films; there was maybe two seconds of fire and Bella internally screaming in BD part 1, and BD part 2 basically starts with Bella's eyes opening for the first time as a vampire (basically the "New" chapter of Breaking Dawn, skipping "Burning").
Everyone forgets that a few minutes earlier she was injected with morphine so she had to wait for the venom to fight off the morphine. She was alive after he injected her with venom but couldn't because of the spread of morphine, which is why in the movie they show her writhing in pain but on the outside she couldn't move. Thus being trapped in her own body.