Intelligence has discovered some very valuable information, but it has always been unreliable. There's plenty of information that is never discovered, and there's an incredible amount of noise that these intelligence agencies are collecting, that the signal is hard to find. To make things worse, other intelligence agencies are purposely spamming everyone else with noise and false information. The really hard part is figuring out what's true and what's valuable.
Agents have historically been fond of generating false intelligence to prove that they are valuable, and historical documentation has shown that agents aren't always that competent.
Using technology certainly helps, but it has its limits. Human intelligence tends to get better information, but can be unreliable and generate lots of false information.
It would not surprise me at all if intelligence agencies failed to realize something is coming. Hindsight shows that the intelligence failures in 9/11 and Iraq were due to valuable information not being identified or the intelligence community sucking up false information from informants who were just making it up to stay in the CIA payroll. It's a real hit and miss sort of business.