This was posted in the fb group from another game. Since the issue is the same the posted solution might help you. Though to be honest I do not think this fix will work for you, it is your missing friends who are no longer active that would most likely need to perform these steps. But it wouldn't hurt for you to try it to see if it helps. But I stand by my belief that the non active friends should stay gone. First, I do not want to have to keep wading through them when working on friend challenges and second they should not have to be bothered by the notifications in DC (and other games) if they are no longer playing. But preaching aside, here is the info:

FYI I just checked my account for some old gaming accounts that were removed from my friends list. I logged back into those accounts through the facebook/change player option and now those accounts are showing up on my friends list again. After testing 5 accounts it is my believe that in order for them to show back up on your list then the fix has to be on their side not yours. So if you want these inactive players to show back up then I suggest you contact them and ask them to sign back in again through facebook.

Thanks for this. It works.

Good game