There are couple scenarios when it can happen.
1. App is not enabled. You can check whether app is enabled by going to "Global settings" page of an app.
2. App can not find unique CSS ID selector to add its content on storefront. App looks for following CSS ID selector, so if you can't find it on your page, you have option to specify your own selector.
#shopify-section-headerGenerally this selector is available on most Shopify themes but it can happen that you have very custom theme and your theme developer decided not to use it. Take following steps to add your own custom selector.
a. Visit "Global Settings" tab once you open an app.
b. Scroll down and you would find "Unique CSS identifier" under "Advanced Settings".
c. Specify your unique identifier without "#" and hit save. Make sure your selector is present on all pages for app to function properly. We generally recommend selector from footer section.
Note: Selector needs to be an ID selector and not a class selector.
3. You've selected app to be enabled only on certain products or products within certain collections and you are checking for popup on a different page/.
4. You are using a theme that has app blocks support so you need to enable app blocks.
Verify whether your theme supports app blocks: Click here
Enable widget or pop-up: Click here