Apps often require some modifications to the theme files to connect to the theme. These changes are either automatic or performed by the app's developers.
Integrate the app with the new theme
If you have added a new theme or theme version to your store, but your apps are not active in the new theme, you'll need to reach out to that app's developers to request assistance with integrating the app in the new theme/version.
For example, say a merchant has purchased the Grid theme and relies on the Product Reviews app. Their published theme displays the reviews, but the new Grid theme doesn't. This means that the integration previously made for (or included in) their published theme is not complete for their new theme. They would need to reach out to the app's developers (Shopify in this case) for installation instructions.