Shared Home and Business 2019 mailbox not syncing for 3 staff

When new email arrives and 1 person works on the email, once work is completed, the email is dragged to a sub folder. This should reflect on the other 2 users screens for that email but doesn't . So other 2 staff has to manually drag that email to the sub-folder. So the syncing isn't working. The same occurs for the other 2 staff. This has been occurring for about 4 months now

Their OST files are the same size of 12.6 gb

Permissions are the same

Each users has the outlook add-in options

Unchecked cache for each and let re-sync overnight and 1 year stored email locally

Used scanpst for each and no issues

Deleted email accounts for each to no affect

If someone has further ideas, am open to suggestions please