Hmm, my experience is that the change in reservation request through immediately so you can approve it. Maybe your guest didn't do it correctly? Did you get any notification at all from Airbnb? If you never received it, perhaps you should make the alteration at your end. Of course you should discuss this with your guest so that they have a head's up to approve the change. If you don't feel confortable doing that you can contact Airbnb so they can take a look and see if the guest's alteration request is pending.
Also, if you are using an app, you might go to a regular laptop, if available , to check for messages. Sometimes the apps are not up to speed. Hope this helps!