On an iOS app, in certain scenerios, users can login with Facebook, others cannot.

Okay, title is broad, but here is what's happening:

When users continue with Facebook, to login to the app, it always authorizes them.

However, some users get pushed back to the home screen and it says that they are already authorized. Basically, won't let them into the app, through Facebook.

This most commonly occurs on iPhones 4 and 4s running 7+. However is happens on iPhone 5+'s even running iOS 8+ occasionally as well.

I'm having a really hard time understanding why I cannot fully replicate this issue. I feel like I am missing something that should be easy.

I would love some help.

Thanks!!