Simple what you need to do is go to factions and make sure you have.
Factions
Currient follower press f2 and type -1 which well enable him to work
Potential follower faction
AI Data
Assistance - Helps friends and allies setting
Remember that you need to pick a voice that will work with the shopkeepers aswell as the follower, might be quite hard as the dialoges are different, your better of experimenting with the voices, I recommend a standard voice would work.
Hope this helps