Cannot Add Boarding Pass to Apple Wallet - Solution

I've seen many people post the problem of not being able to add their airline boarding passes to Apple Wallet. The "Add to Apple Wallet" button occasionally disappears. At least this happens in the United app, not sure about the others. I think I found the solution:

The reason the "Add to Apple Wallet" button is not showing is likely because there is a hidden expired pass in Apple Wallet similar to the one you are trying to add.

Do this on iOS:

1. Settings -> Wallet & Apple Pay

2. Scroll down and turn off "Hide Expired Passes"

3. Go into Apple Wallet and manually remove all the expired passes that are now showing

4. The "Add to Apple Wallet" button should not be visible in the United app

I suggest Apple turn off "Hide Expired Passes" by default.