I think there is a service running on the Android devices which periodically(not sure what that interval or time is) checks for upgrades.
1. Have you waited long enough to conclude that upgrade-check was made but not found?
2. Any chance that this check-for-update-service has been disabled on your handset?
3. Have you
versioned your app properly?
4. Did you
change the version name and version code in the manifest file when publishing upgrades?