How can I stop some calendar entries from disappearing

For some reason. most of the appointments I make for my massage therapist, disappear. But only if I make them at the clinic. When I am there, I enter the next appointment.I then check to see if it is in the calendar. And it always is. However, every one of them (except once) disappear a few days later. I am not deleting them. even accidentally because all other entries are always good. Is there a way I can stop this from happening? Note - the mobile reception there is dicey. I have Samsung **Note 20** and **Business Calendar**