TL;DR:
- Voldemort didn't understand the power of a mother's love and how it could put a stopper in death (as Snape would say)
- Voldemort thought himself "most important and precious"; it would not occur to him that Narcissa might value Draco's life more than his.
- Voldemort never for one second imagined that Narcissa would dare to lie to him.
- Not understanding he had bound his own life to Harry's when he took Harry's blood with Lily's enchantment to use in the resurrection potion, Voldemort had no reason to think Harry would survive Avada Kedavra. He was overly confident of his omnipotence and woefully ignorant of the power of love and Lily's enchantment.
- Voldemort was revolted by and feared death; he would not approach a corpse, much less touch one, for the purpose of determining life. He would have wanted to believe Narcissa.
- Voldemort hurt Narcissa; combined with the hurt and pain Voldemort had caused Lucius Malfoy and Draco, Narcissa may have been so angry that it spurred her Occlumency (if she employed it) to be stronger.
I keep revisiting Voldemort's overall mindset and psychopathy.
But surely if the boy had destroyed any of his Horcruxes, he, Lord Voldemort, would have known, would have felt it? He, the greatest wizard of them all, he, the most powerful, he, the killer of Dumbledore and of how many other worthless, nameless men: how could Lord Voldemort not have known, if he, himself, most important and precious, had been attacked, mutilated?
Lord Voldemort - Deathly Hallows - chapter 27 - The Final Hiding Place
What Voldemort didn't understand is a mother's love for her child. He, as Tom Riddle, was not enough to keep Merope Gaunt's will to live alive. No matter how odious one might think Draco Malfoy is, Narcissa is Draco's mother, and she loves him beyond any measure. Her concern is no longer for Voldemort, the war, the Death Eaters, or her side winning. Her sole concern was for the safety of her child, her only child who, remember, Voldemort sent on a death mission, fully expecting Draco to be killed, in Half-Blood Prince.
I deduce that Voldemort, not understanding love at all, much less the specific love a mother has for her child(ren), never for one second imagined that Narcissa would dare to lie to him about something as hugely significantly as whether or not Harry Potter was alive or not. As noted in the quote above, Voldemort thought himself alone "most important and precious"; therefore, I don't think it occurred to him that another might not feel the same way, that someone else might value another person more than they valued Voldemort. I feel this is highlighted by this next passage:
‘And his knowledge remained woefully incomplete, Harry! That which Voldemort does not value, he takes no trouble to comprehend. Of house-elves and children’s tales, of love, loyalty and innocence, Voldemort knows and understands nothing. Nothing. That they all have a power beyond his own, a power beyond the reach of any magic, is a truth he has never grasped.'
Albus Dumbledore - Deathly Hallows - chapter 35 - King's Cross
Not understanding that he himself had bound Harry to life when he took Harry's blood (with Lily's enchantment in it) in Goblet of Fire, Voldemort would have had no doubt that he had killed Harry when he cast Avada Kedavra and Harry fell to the forest floor.
‘Think back. Remember what he did, in his ignorance, in his greed and his cruelty.’ [...] ‘Precisely! He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily’s protection inside both of you! He tethered you to life while he lives!’
Albus Dumbledore - Deathly Hallows - chapter 35 - King's Cross
Voldemort was revolted by and fearful of death; he would not approach a corpse, much less touch one for the purpose of determining life. He would have wanted to believe Narcissa when she said Harry was dead.
‘You,’ said Voldemort, and there was a bang and a small shriek of pain. ‘Examine him. Tell me whether he is dead.’
Deathly Hallows - chapter 36 - The Flaw in the Plan
Voldemort hurt Narcissa physically. Coupled with what Voldemort had done to Lucius Malfoy and Draco and physical pain, Narcissa may have decided to employ Occlumency. The only issue I have with the Occlumency theory is a skilled Legilimens can tell when someone is blocking them out via Occlumency.
'[...] don’t look at me like that! I know what you’re doing, I’m not stupid, but it won’t work – I can stop you!’
There was a pause and then Snape said quietly, ‘Ah ... Aunt Bellatrix has been teaching you Occlumency, I see. What thoughts are you trying to conceal from your master, Draco?’
Deathly Hallows - chapter 15 - The Unbreakable Vow
I tend to think that Narcissa was not employing Occlumency.
I think that Narcissa acted on impulse, that it wasn't something she even thought about doing until Voldemort hexed her and told her to go check Harry for signs of life. Between where she was sitting and the time she reached Harry's body (probably mere seconds) she decided that if Harry were alive, and could confirm Draco was alive as well, she would lie to Voldemort. When she did, Voldemort did not sense it was a lie for the reasons outlined above, nor did he try and check whether Narcissa was lying or not.
Yet another example of Voldemort ignoring the obvious in favor of what he valued.