Why don't the letters appear after the year for ambiguous citations in the bibliography?

It appears that the letters for ambiguous citations will only appear in my bibliography if I insert the ‘year’ field into my in-text citations.  I can’t do that, because I’ve had trouble trying to get it to do author (year) sometimes and (author, year) other times.  The best solution I can think of is actually putting the letter in the year field to begin with, but I’m not happy with that hack.  Can anyone suggest a better solution for me?