Answer:
This question may refer to the exelent TED talk of Phil Plait: How How to defend Earth from asteroids.
In it, he says that the problems may be timming: Because you should detonate the nuclear bomb in the exact time (with a tolerance of miliseconds) to move the asteroid, which is a really hard thing because the asteroids move really fast, and if you detonate the weapon a few seconds earlier or latter, you will miss the asteroid.
