I have had luck recently with closing the drawing and when I reopen the drawing and open a fab sheet they show up. They have also disappeared while editing a drawing. I close everything then open it back up and they appear. Not a fix but it does help.
I have downloaded Advance Steel 2015.1 and used up the trial period. As I understand it this is an update if you have a subscription. I have tried to to activate it but it keeps telling me that my product code is wrong. Please advise.