Mail not adding addressees to prev recip list

I'm running 10.6.8 on an older MacBook. Mail v. 4.5, Address Book v. 5.0.3. Lately I've been having a problem in adding addresses to Address Book. It simply doesn't work. It used to be that Address Book would open and prompt me to enter name, etc. for the new entry. Now what happens is that I select "add to address book" from the address in Mail and it shows the name in the address bar of the email as if it was entered, but the next time I open mail and attempt to type that person's name it won't auto-complete, and the entry never appears in Address Book either.

I've gone through my Previous Recipients list and added those names I want to add to address book. Those names are not appearing in Address Book or in Previous Recipients. Even names to which I have written many emails are not appearing in Previous Recipients and do not auto-complete. This is a relatively new problem.

I did go into Preferences in Address Book and check to see which account it is associated with. It's On My Mac - Local. There is only one user account on this machine. I used to have two email accounts associated with this user account, but deleted the second one after a while. I'm wondering if that is connected to the problem.

I've looked through the forums but haven't found this problem... any help would be appreciated.