Terrorist info ignored, too much data or security failure?

We hear that Russian intelligence officers had warned both the FBI and CIA about Tamerlan Tsarnaev, one of the two Boston bomber ‘suspects’.
How often do we get info like that? Do we get info like that once in a blue moon but it was just ignored by U.S. security officers? Do we get info like that thousands of times a day, so who could have picked the important one out of the noise?

We know that there was a security brief before 9/11 2001 that observed that AlQaeda were planning to fly planes into buildings. How often is such an alarming memo produced? Are there a dozen more every day so it’s hard to tell what’s the problem that should get attention? Do they occur with such a dire message once a month, once a year?