When uploading a newer version of an APK file, the upload completes successfully, however, the version of the application does not change in the Mobile Device Manager console. As a result, the update is not pushed to end-users.
The Mobile Device Manager console is able to read the version used in an APK file by checking the versioncode specified within the APK file when it has been built. Information on how the version code may be verified and changed when a build of the app is being built may be found here
To address this issue, one would need to ensure that the APK file being uploaded uses a newer VersionCode.