I had the same problem on iPad... I figured out that it was on Amazons side because the preview covers of mailed in docs that the app used to generate were gone as well, a complete delete, reboot, reinstall helped solve the problem (for now, i had it before, it all started with version 3 of the app)... But i suspect you cant do that on the Fire.
The customer support had to following to say on the issue:
"We can't restored back the feature on your device."
Whatever that means... (for a company that is dealing with books, this is just embarrassing)
I suspect they dont care to much about the whole issue because what would you mainly want covers for? Books from other sites, which is money lost to amazon...