We all send out emails that seem to go by the wayside. If you want to track down those emails still waiting for a response, you can use a Google Apps Script.
The script searches through your Gmail account and looks for emails you sent that didn't get a response. The script then adds a label to those emails so you can easily find them and decide if you want to send another reminder email. You can grab the script here and get the installation instructions below.
Find Unanswered Emails with Apps Script [Google App Developer Blog]