Appointments added in Outlook on my Mac don’t appear when I save them but do appear in my synched iPhone and iPad?!?

Sometimes….but not always I add an appointment to Outlook on my Mac and when I save it there's no sign of it on the calendar on my Mac…..but I can see it on my iPhone/Ipad.    This may also be linked to another problem where I can get 2 or 3 reminders pop up at same time for the same calendar entry.   Any thoughts anyone?