During app installation it adds some code to your theme's templates. If you installed a new theme after the app, it will miss the required code and app will not work.
The easiest way to fix that is to go to Apps -> EasyTabs -> Installation -> Click the "Enable" button next to the title of the theme.
If it doesn't help, you may grant our support team access to your Store Admin - we'll add the required code to your templates manually and make sure app is working properly. You may send an invitation to firstname.lastname@example.org. Please don't forget to send us an email explaining your problem.