Why did the Necrons not awake at same time during The Great Awakening? Some awoke decades before others, and some Tombs never awoke to begin with. If Awakening was more synchronized they could have conquered the Galaxy with relative ease.
Quoting from the official Warhammer 40k wiki
For 60 million Terran years the Necrons slept, voicelessly waiting for their chance to complete the Silent King's final order: to restore the Necron dynasties to their former glory. As the centuries passed, ever more Tomb Worlds fell prey to malfunction or ill-fortune. For many, the results were minor, such as a disruption to the operation of the Tomb World's chronostat or revivification chambers, causing the inhabitants to awaken later than intended -- but some of the Tomb Worlds suffered more calamitous events. Cascade failures of stasis-crypts destroyed millions, if not billions, of dormant Necrons. Some Tomb Worlds were destroyed by the retribution of marauding Eldar, their defence systems overmatched by these ancient enemies of the Necrons. Other Tomb Worlds fell victim to the uncaring evolution of the galaxy itself. Tectonically unstable planets crushed Necron strongholds slumbering at their hearts; stars went supernova, consuming orbiting Tomb Worlds in their death throes. And everywhere, inquisitive lifeforms scrabbled and fought over the bones of Necron territories, causing more damage in their unthinking search for knowledge than the vengeful Eldar ever could.
Basically what happened is time happened, a lot of things changed around them while they slept, and even their advanced technology (which in it's peak rivalled that of the Eldar and the Ancients) wasn't immune to the damages of time, their chambers and machinery fell to disrepair, went out of tune or destroyed all together. in 60 million years a lot could happen.