These days too many of them are a bit intimidated by the mere presence and aura of people in uniform, intel people, cops etc...Years ago, many of them had served in the military or a had a very good understanding of what they can really do (not what they claim they can do) how they worked/the types of people behind them(JFK, Churchill etc) so were rightly sceptical of their claims. I mean, has anyone ever met a cop/spook/politician who didn't want more power to do X, more money to do Y, more people, more secrecy etc?
One of the main problems for intel agencies is that they are merely one conduit to policy-makers, amongst many these days. Ultimately it is those policy-makers who need to (and increasingly are incapable of) using this information for strategic decision-making...Tactically it is easy to say "arrest/shoot that guy," because they get kudos and less blame if some thing goes wrong. Strategically it is harder to say "don't shoot that guy, keep him alive and in 5 years we can negotiate with him." It took the British a long time to learn how to do this in Northern Ireland, and it was only when they did that they managed to start to move towards peace with the IRA. I'm not sure would that be possibly in the bloggy, politically polarised, short-term, tabloid, Sky/Fox News type of world we live in these days.
After all, even if they did predict the rise of ISIS, Boston, it is the policy-maker who must decide what action to take. Difficult when:
a) There are sooo many warnings coming across their desk
b) Limited resources
C) Not many real policy instruments available. For example, the US public would not have been up for attacking for acting on intel available about ISIS 18 months ago, thus that policy option was thus not available.