App won't update?

[MizMelzy76]

I downloaded an app from outside of google play. Google Play also has the app in their store. When I go to the app page on google play, it shows its installed. It's not listed in my apps though, but google play sees that I have it installed. I know that there is an update for the app, and a page for it on Google Play. When I go to the page it says "open" but does not show the update. I have it set to auto update, but it doesn't. Is this because it was not downloaded through google play? Is there away to fix this other than uninstalling and reinstalling from google play?

Thanks!
Mel