We also arrive 7/31, but the Disney dining reservation website is only letting me make ADRs for that day, not the +10 (despite being on property and the reservation being on my account and all).
The CM I spoke to said the +10 is working for some people but not everyone. No explanation as to why that is. My reservation is a DVC one & she thought maybe that was why. I was able to link our room reservation but couldn't book the +10. She did it for me over the phone & everything show up in one place.