Emails delayed showing up in Outlook Inbox

Hi

I hope someone can offer a few thoughts. I'm having problems with emails arriving in the Inbox for several clients.

There are 5 people accessing a shared mailbox in the same physical office. Some have the mailbox setup as their main account whilst others have multiple accounts all setup as individual account, not accessed via share permissions.

Clients are using a mix of Outlook 2010 and Outlook 2013. All operating systems are Windows 7 64 bit fully updated

All systems are running ESET Endpoint Antivirus

Mailboxes are hosted on Office 365

For one user emails arrive with no issue, user is running Office 2010

For the other users an email arrival notification message will be shown and the mail can be opened from the notification. You can then reply to the email and replies will show up in the sent items. After closing the email it cannot be seen in the Inbox.

For some users the emails will all finally arrive in the Inbox hourly in a batch. Other users could go a whole day with nothing arriving.

Things I have tried or tested

Rebuilt all of the Outlook profiles
Tried with and without cached mode
Tested as single mailbox, primary mailbox with additional mailboxes and as the secondary mailbox
Reinstalled office
Tested internet speed and access to mailbox
Tested through OWA with no issues
Disabled email scanning on ESET
Fully disabled ESET AV
There are no rules setup in any of the Outlook profiles

There are other teams in the office accessing similar mailbox setups with no issues

Any ideas about a fix would be most appreciated