Make sure you have your view distance set to "ULTRA" when possible while hunting for rares. If your video card can draw an npc your NPCScan will go off. If your settings are too low you could potentially miss rares you would have otherwise found.
When you're ready to begin the hunt for rares NPCScan should look just like the one in the screenshot below. Notice how there are no red x's under the cached column. If you see a red x next to the particular mob(s) that you are looking for, follow the steps above to clear your cache.
One Other very important piece of information I'd like to add is IF YOU TAB OUT OF THE GAME YOU WILL NOT BE ABLE TO HEAR NPCSCAN IF IT GOES OFF IF YOU ARE NOT IN WINDOWED MODE. Even if you have "Sound in Background" checked off in your options menu. If you're planning on sitting your character on a spawn point or patrol location of a rare and then tabbing out and browsing the web, this is not an option. I learned this the hard way when I was camping for Aeonaxx on my husbands toon. He had tabbed out of the game to research something on the web. When we got back into the game Npcscan's animated Aeonaxx button was right there in the middle of his screen, the alert was going off, his screen was flashing red and general chat was all about who downed Aeonaxx. We won't make that mistake again... In windowed mode however, you will be able to hear it if your tabbed out, it's in the background, and it does go off.