App not found in App Store by some users, but found by others
Actually, there are several possible reasons:


Not available in the user's country
The AppStore take some time to deliver an app update to everyone (should be available within 24 hours)
Not available for the user's device (32bits, iPad or iPhone..)
Phased Releases option used in AppStore Connect


This list is certainly not exhaustive*

- How to browse iPhone App on iPad:

http://osxdaily.com/2017/10/18/download-iphone-apps-onto-ipad/


- 32 bits apps aren't showing up in the AppStore anymore:

https://www.gizmodo.com.au/2017/06/32-bit-apps-arent-showing-up-in-the-app-store-but-dont-panic-yet/