Too much data
Indeed, with the Charlie Hebdo murderers having been under surveillance that was later discontinued, it seems that we are seeing the rise of Big Brother states which cannot cope with the volumes of information they greedily seek access to.
We reached that years ago. The Americans knew about the Sept 11th terrorists. Same in the UK with the Lee Rigby murder.
This story on the BBC's More Or Less program deals with this exact problem: How to deal with all the information to spooks collect. In it, Stella Rimington (former head of MI5) says that even if they know every terrorist suspect, they can't monitor them all as it would require too much man power.
Another problem (as highlighted by Robin 12 above) is false positives. If you have too much data you're more likely to finger innocent people as terrorists.
The spooks don't need MORE data, they need to work smarter with the smaller amounts of data.