It was explained in the books somewhere, sorry that I don't really remember where... But they discussed the fact that Fain had come very late the year before and it was stated that this was because he'd been taken to SG to be 'prepared' then he'd had to spend extra time in each village checking out the likely boys. When he finally got to the Two Rivers, he spent a longer time there than normal and seemed overly interested in Rand, Perrin and Matt. It was the following year when Moiraine and Lan came that Fain returned with the shadow spawn.