I have had this happen several times, always with reservations that start with the digit 9. Marriott reservations start in the 700 range for the first 3 digits and then the system moves up through 999 before restarting again at 700. The missing reservations are always in the 900-range of reservation numbers. I have had at least a dozen of those. The property always has the reservations and if you actually search for the reservation using the reservation number, last name and check in date, it will show up on the website, but indeed not in your reservation list.