Here's an annoyance with the "Past Stay Credit Request" form:

For a recent stay, I was missing the G3 and the Diamond Amenity bonus. The form only lets you specify one bonus code, so I entered "G3" and in the comment field I explained that the Diamond amenity didn't post either. Sadly, that comment was completely ignored and only the G3 bonus posted.

So I tried to submit another request, this time for the 1W Diamond Amenity. But that doesn't work -- the website rejects a second request for the same stay outright with the error message

"You have already submitted an uncredited-stay request for these dates at this hotel. For assistance, please contact Hyatt Gold Passport Customer Service at 1-800-30-HYATT or your nearest Worldwide Reservation Center."

I know I can just email GP Customer Service, but IMO it's still a poorly designed feature.