Guest Codes Not Working

I'm new to Ring, and just installed a system in a house we're getting ready to sell. My problem is that I've set up guest codes for the real estate agent and a couple of other people, but I can't get the system to respond to the guest's access codes. I've done everything I can think of, including setting schedules for the guests (all day and all seven days).When I try to test the system using one of the guest codes, the Ring app reports the incident as "PIN Rejected". Did I miss something, or is there something wrong with my system? Thanks a lot for your input!Mike