Immortals can only sense pre-Immortals if they have a certain degree of compatibility. So, for instance, if Duncan had been holding some sort of Immortal party when Richie dropped by, only two or three of them would have been able to sense him. This pre-Immortal sense also draws at least one compatible close when it's close to the time for the pre-Immortal's first death. Of course, even with some amount of compatibility, it doesn't always work out well, and after the first death there's nothing to say who's compatible or not, so it's a bit of a crapshoot, but at least there's more of a chance and unscrupulous Immortals can't go around finding all of the pre-Immortals, killing them, and taking their heads.