Android Play Store installs previous version of app in Alpha

I have an Android app in alpha testing; I have uploaded updated APKs a few times. My testers report that they still get the previous version, not the latest one, even though the Developer Console says the latest version is the right one - the latest I uploaded a few days ago - and doesn't say "Processing update" any more.

Is there anything I should advise my testers to do on their devices, and can I do something at my end?