Play Store having new version of the app, but not showing update

Make sure the app you have installed in your mobile is not an apk built with debug keystore. The play store will show always as "Open" button instead of "Update" if the play store app certificate doesn't match with the installed app certificate. I realized this when I did trial and error with debug and signed apk's only signed apk worked. And always use versionCode for comparing versions instead of versionName.