Are you checking the version number both before and after "updating"? I ask because I have come across places that sometimes show your current version in the store until after you update.
Well, it still shows the current version after updating, but updating changes the current version.
But the real important detail is that all Android apps have a release/build version code that must be higher than the previous one for the Play Store to accept it as an update. This number can be different than the publicly displayed version number. Make sure you're comparing the correct number when checking for updates.