Listen to this (long wallotext) :
There are three groups of Pokemon in the PokeWalker : A, B, C ! There are two in each as follows : rare (A), uncommon (B), common (C) . In the Yellow Forest, A contains Flyer, Surfer B contains Flailer, Volt Tackler and C has regulars . But here comes the tricky part ...
When you import your Pokemon to the 'Walker, it will choose only ONE Pokemon from the groups . This means that on a single stroll you can't get both and this makes it a pain ... :o
I hope this was valuable and helped ... it helped me
Note : I didn't copy paste ! I raw-written this !
Source : serebii.net