There are many potential reasons why an insert will not appear when using your app:
Audience: You may not fit the audience criteria. For example, if you have a device configured with a locale of French and the audience of the insert is set to English, you will never receive the insert.
Capping: You may have exceeded the capping rules. Capping defines the frequency at which the user should see an insert. Capping can be once per session (app launch), once a week, once a month etc. Most inserts also have a goal capping option that means an insert will not be shown after a goal is achieved. For example, if you answered a survey, capping rules may prevent it from being shown again.
Trigger: You have not performed the appropriate trigger. Some inserts do not appear until a user views a specific screen or taps a specific element.
Network problems: If you have network issues, you may not see inserts. For instance, if there is no connection at app launch, the inserts will not be retrieved. If it takes too long to retrieve images in an insert, the insert will not be displayed.
Clash: When there are multiple floating or fullscreen inserts defined for the same trigger and audience, only one will be displayed.
It is recommended that you always test inserts using the Test on Device option before publishing. You can read more about that testing feature in this article.