Thanks for the update. If it happens with some senders, are those emails going to Junk or another folder?
In New Outlook for Windows, sync settings are different from classic Outlook. There is no “download for all time” option. Your mailbox stays synced with the server automatically.
Please check these steps:
• Open Settings, then Mail, then Rules. Make sure no rule moves emails from those senders.
• Go to Mail, then Junk email. Check if the sender is blocked. Add them to Safe senders.
• Turn off Focused Inbox under Mail, then Layout. See if emails appear in Other.
• Ask the sender to confirm the email address is correct and not bouncing.
If emails still do not arrive, sign in to Outlook on the web at https://outlook.live.com
using the same account. If emails are missing there too, the issue is server side and Microsoft Support needs to check your mailbox health. If they appear on the web but not in the app, use Settings, Accounts, then Repair to refresh the connection.