Using Outlook for iOS for personal email and calendar management
You may check if your email account was configured to mail app as POP, POP account cannot sync custom folders, and also if you configured email account to Outlook as POP, and then create folders there, in this time the folders cannot be synced either.
So better check them from your email web at first make sure they are synced to server side, if so, then you could configure your email account to mail app as IMAP then see if you could see the folders.
Hope it helps