The following is a useful troubleshooting guide that's helped many with this query:
Head into Settings > Apps > Email > Storage > Clear cache, then Clear data. Once this has been done, you would need to sign back into your email account. After you’ve signed in, make sure ‘Sync’ is enabled. You can check this by swiping down from the top of the screen > toggle on the Sync icon From here, head into the email app > Menu (3 lines at the top left corner) > Settings > select the account you're experiencing issues with > Ensure Sync account toggle switch is on orange. Furthermore, I'd also advise heading into tap Sync schedule > set Sync schedule > Auto.