Exchange Email Not Showing Up on iPhone

I'm not getting Microsoft Exchange emails on my iPhone 8. Emails that appear in the inbox on my desktop version of Microsoft Outlook 2011 do not appear on my iPhone Exchange email inbox (or anywhere on the phone). Same with some sent emails. The problem seems very erratic and consequently is very frustrating (emails from the same address sometimes do and don't show up). Sometimes I'll discover hours (or more) later that I have received email which does not show up on my phone, which is my primary method of checking email. Since I use Microsoft Office 365 Email and an Apple iPhone as my primary contact with my customers, it makes me look very unprofessional when I am not getting emails on my phone (which is honestly the most important feature to me - even more important than making phone calls). This has to be fixed as it is making me look bad! Thanks in advance for your attention to the problem.