A surprise with Word on the Mac
If you have two works from the same author in the same year, then many citation formats designate one 'a' and the other 'b'. If automatic updating is turned off, these suffixes are added by Zotero when you click the update button. But if a citation has been modified, the suffix is not added.
It's almost never necessary to modify citations in the text, and there's no way for Zotero to keep citations updated if you do. See Customizing Cites for the right ways to do this.
You can reset citations you've modified by clicking on them and clicking Add/Edit Citation, at which point it should prompt you to revert them.