It's common during writing to make changes to your reference list; and Zotero can update your exported bibliographic files even after they have been generated. Navigate to the Automatic export tab. Open the 'Automatic export' drop down and select 'On Change'. This commands Zotero to update the exported bibliographic file as soon as you make any changes in the parent reference library in Zotero. This option works just fine in most cases; however, if you tend to do a lot of heavy processing with your computer or are working with old hardware, you may want to select 'When idle'.