If you have installed a Cordova plugin and it is not working on iOS, for example the expected JavaScript object for the plugin simply doesn't exist. But you have tested and find that the functionality is working fine on Android, this is usually a symptom of an error in the iOS plugin class target membership.

For example this can often happen with the Cordova SplashScreen plugin. See the screenshot below and ensure that the .m implementation file has a checkmark on the "Target Membership" option for your application.

XCode target membership preview

Only the implementation file should have the target membership checked (so not the .h header file).

This is often the reason why a plugin appears simply not to work. It's always good to check this setting first before debugging further.