Every character I met that the guide says will join me does not. It always says something like, you are not experianced enoughf for me.
You have to raise your level before they'll join you. For example, if the NPC's level is 12, they won't join you until you're at least level 11.
To get them to join, you must be either above, at or 1 level below theirs. A level 35 character is not going to follow a level 6 character, plain and simple. You also have to have certain requirements, sometimes you must be evil, other times good, have high enough charisma or perform a certain quest.
Well, you've forgotten about alignment... For example, Magnus won't follow an evil character, and so on.
Milo, from what I've read at the AI, its always two level short, not always, but most of the followers. If Vollinger is 15 you need to be 13, etc.