Well, obviously there's some glitch making what should be pretty straight-forward a messy hassle. Perhaps it's time to just start over doing the calendar sync process completely. Maybe try this:
-- On your phone, go into your Settings >> Apps menu, find the Calendar app you want to use and open its entry, tap on 'Force stop' and then find and tap on 'Clear data'. Using 'Clear data' will wipe that app's settings and its cache, basically returning it back to its first-time used, yet-to-be-configured state.
-- Open a web browser and go to:
https://calendar.google.com
and log into your account (Which is presumably also the same calendar you want to sync with). Check that the calendar (if you have multiple calendars set up) that you're trying to sync with is selected in that left side panel.
-- Now launch the Calendar app and try to sync up with it again. Don't forget to check in the Settings >> Accounts >> Google menu that syncing is still enabled and that in the Google >> Apps menu that in the calendar app its Permissions is allowing Calendar access.
One last thing to check is your online connectivity. Syncing won't happen if your Internet access is the problem. Are you using predominantly WiFi or mobile data (cellular), or just use both most of the time? Are both pretty much stable and reliable?