I didn't get my email within a couple of hours of a recent stay, so I used the chat facility on the app to request it and within minutes of them acknowledging my message, the email was (re)sent.
I know last year when I was in NYC I didn't get the email after checking out of the Marriott Marquis, so I used Facebook to message them and I was able to get it that way.
Previously I have called a hotel up too, to prompt them if the email hasn't arrived.