If above solutions did not work for you then you may have doing something as following ..
1) installing the app from Appstore.
2) updating it with sign APK with same package name updated version.
So basically there are two kinds if APK's.
1) you uploaded on playstore known as original APK.
2) download from playstore known as derived APK.
In this case basically you are downloading derived apk and updating it with original APK.
For let it work fine uploaded new signed released APK in the internal test mode on the Google Play Store and download the derived APK to check the update scenario.