Every now and then whenever you try to wack someone you get a message You could not find [Insert name here] Perhaps you should consider buying a hit on them the only thing is you know where they are...

Yeah, it happens when the guy you're trying to shoot has been inactive for some time. I'm not sure of the exact number of hours though. The workaround is to simply add the target to the hitlist. You can find it under Actions -> Attack -> Hitlist