Some or all emails are being received in a different folder in Gmail ( spam, sent, all mails) but not the inbox.
You must check for filters.
- Go to Gmail.
- Click the Gear icon at the upper right then select See all settings.
- Go to the Filters and blocked addresses tab.
- Review filters listed and delete the one causing the conflict.
- Save the changes.
You can also check if a filter is affecting the email by getting a message ID and look the affected user in Google Admin
using the Life of a message
and Filter replayer
This is cause due to filters configured in Gmail or other clients emails, if you are using client emails with an IMAP connections, filters or rules configured in the client will also affect emails in Gmail web interface.