Google decided not to allow developers to do it.
If you have this message in market it because you are trying to install app on old or not compatible device (like Tablet PC). Nights Keeper have only two requirements to the devices: Android 2.2+ and telephone functionality. Google Play says not compatible with my device. The back size of this could be wrong behavior of all time based apps, like Nights Keeper or Alarms. So, you phone changing time, date or time zone like in hell.
It could be spam filters or external sms messengers (for example GO SMS PRO or Handcent). Sometimes it doesn’t work because of conflicts with other software. Please read section below for more information. If you have dual sim phone then it could be a problem. Turn on SMS sound notifications, but it doesn’t work.