Yesterday I was in the bus with my wife and I asked her if she received a calendar invitation, she did but the event didn't show up in her Samsung S Planner calendar. She said that normally, when she accepts an invitation it shows up*... she clicked "yes" again, still no event in the calendar. I checked in the app if there was an option to fetch the updates, I found one called "Sync now", I touched it. With show, we noticed that all her past and future events disappeared. My invitation was there. So what happened?
My wife has a Samsung phone (Samsung Galaxy S5 mini) with a Google account. For some reason she doesn't like the Google Calendar and for another strange reason (maybe to save some data traffic**) she deactivated the synchronization with the google account. She also hasn't a Samsung account. So basically she has no synchronization with any online service, everything is local. Some entries (like my invitation to an event) appear in her calendar because the S Planner app shows the google calendar entries.
I believe that the S Planner app is so buggy that if it doesn't find a Samsung account and one touches "sync now" it simply sync from an empty calendar which the app thinks it's the "master". So if you sync from master to slave, slave gets all updates from master, thus in this case everything is lost. What a stupid buggy app, really! A synchronization feature should be smart enough to sync in both directions to avoid data loss like in this case.
Anyway, I suggest people not to trust apps and always do backups. How do I do backups? Well, I synchronize my important stuff (contacts, calendar, notes) with the cloud so if my phone get broken, stolen, lost etc. I can simply open a browser anywhere else and see my data.
Another app I use, for all those apps that save data locally on the phone is Titanium Backup: I scheduled an automatic backup every morning a few minutes before my alarm for all my important apps and kept a few versions, just in case (so e.g. if an app erases everything and I notice a few days later I can still restore an earlier version, not just the last useless one).
So: activate that damn sync and use it (Google, or Samsung, or both). And if you still wanna use buggy apps such as S Planner, make sure to make backups of your local data if you don't synchronize it with any cloud.
Open your Android Settings > Accounts > Google > touch your Google account > make sure "sync" is on > you can select and deselect what you wanna synchronize.
*You can activate the Google account synchronization and selectively choose what to synchronize. E.g. if you backup your pictures manually you can deselect photos, but sync at least contacts and calendar and other data that matters to you.
**I believe my google calendar event invitations showed up when she accepted them from a browser, so e.g. logged in in gmail, when clicking yes, gmail would update her google calendar.