title: Unable to Track Field History of Custom Fields in Custom Objects
body: <p>I have a custom Object called Billing and a number of custom fields in it. 
  What I'm trying to do is enable Tracking Field History so that whenever one of these Custom Fields is changed, the change will show up on the history log. 
  So I went to Setup -> Create -> Objects, select Billing from the list of Custom Objects, select Set Tracking History and check the boxes besides the required Custom Fields.</p>
  
  <p>The problem is that only changes in one of these fields show up on the log, while the rests don't.</p>
  
  <p>Any idea what's wrong? Thanks.</p>