Referee the Defense

[ATLBrvs326]

I am watching some NBA games tonight and thinking about referee the defense. This has been what I have worked on all year long as well as thinking the entire game on the court (not being dead minded). Anyway, I have noticed that sometimes I miss calls when I referee the defense, especially in the paint when players are bundled because if you referee the defense when a player is getting guarded by 3 players, your more than likely gonna miss someone foul the offense. Watching the NBA game, I see the officials watching the offense sometimes, while other times watching the defense. So when exactly do I watch the defense, and when do I watch the offense. In the example above, would I watch the offense and watch for fouls? Any advice would be great.
Travis