Neither guest nor host can alter reservation dates from June to Sept

Hi, a guest is asking to change her reservation from 3 nights in June to 3 nights in September because the event they are coming for has changed its dates. When I go into to alter the reservation, it tells me that the dates are not available even though they are open on the calendar. The guest is seeing the same thing when she tries too. I have revisited the calendar to make sure those dates are available - they are. Any clues as to what might be happening? We have a strict cancellation policy, but I don’t get the feeling she is trying to subvert that. Anybody had this issue?