This is a list of known bugs :

#) When the app is bought and launched, it is not possible to start a game :
When the app (Full version) is started for the first time, it checks online if it was downloaded from Google Play (license checking, for copy protection); after, it will check one time every 3 months contacting Google servers.
The procedure is well tested and used by a lot of different applications, but in very rare cases there may be problems with Google servers, or network problems.
Fortunately, only 3 failures have been reported in more than 12 months.
If this happens, here are some solutions :

1) Go to Android Settings > Apps > Google Play > select “Force stop”, then “Clear Data” and “Clear Cache”.
2) Go to Android Settings > Apps > All > Google Services Framework > select “Clear Data” and “Clear Cache”.
3) Go to Android Settings > Apps > All > Google Play Services > select “Clear Data” and “Clear Cache”.
4) Make sure that the primary account on your device is the same account you used to purchase the game, check this via Android Settings > Accounts > Google.
5) Make sure that your system date and time are correct : Android Settings > Date & time > enable “Automatic Date & Time” and “Automatic time zone”.
6) Try signing into https://wallet.google.com/manage to verify that the app is not “refunded” nor “cancelled”.
7) Check if you have other apps installed which can interfere with the standard Google Play Store, try to remove them to prevent conflicts.
If you have installed root utilities for removing licenses from apps, they will cause problems because they emulate the standard Google Play License verification process.
8) Uninstall the game, reboot the device, download and install it from Google Play.
9) Check if Google Play Services is updated, in the Google Play Store.

#) U-Boat motor sound not played :
When the app is launched, sometimes the sound of the U-Boat motors is not played :

Solution :
Check if the Enable sound option is selected (in the Option screen), then try to deselect and select it again.
Or try to close the app and relaunch it.