S planner and Macbook Calendar

[AC Question]

I've never ever had problems syncing between my Macbook Pro's calendar and my S Planner on the phone but just today found that an event I keyed in 4 days ago didn't sync and I missed an appointment. In those 4 days I was on my computer more than once using the internet etc, so I imagine there would have been enough time for it to resync if there were issues initially?

Please can someone help/explain in case it ever happens again.