Q: I do not get the pop up message when I open the app link on my iphone?
A: The install prompt will appear on IOS devices if you are using safari, or for Android devices, it will appear if you are using Chrome or Firefox. Android has numerous browsers like dolphin that doesn't support installing apps on the device.
If you are sending the link via a facebook message, then it won't work. It must be opened with Safari on IOS devices (Chrome on Android) in order to get the install prompt. Facebook has its own browser that does not support installing apps on mobile devices so you do not get the prompt.
The install prompt only appears once... the first time the app is opened on a phone and will disappear after a few seconds if a user doesn't do anything. So it is possible that the person missed the install prompt. In that case, you will need to provide them instructions on how to install the app.