To try and force restore your progress once again, please play a game until the end while you are connected to a WiFi network and to Google+. After this, please press the Restore Progress button in the settings again.
If this is still persisting, make sure you are using the same Google Play email address used before when saving data.
To learn how to see if you are using the correct email, click here.
Unfortunately, there have been reports of users that were not able to restore their progress despite following the steps correctly. This usually happens when the purchases were made whilst the user had installed older versions of both the Plague Inc. and the Google Play Store App.
We are always hoping to improve and with each update prevent this problem from re-occurring. Sadly, even if a permanent solution is rolled out it won't have a retroactive effect and your progress will not be restored. Fortunately, the awards on the Play Store will remain there.