Mail mailboxes not showing.,

This must have been answered before but I cannot find it.

we have three computers. All on iCloud, all synching correctly.

On two of them the mailboxes show in mail correctly. All three show the mailboxes on the internet iCloud connection web site.

However on one of them they do not show at all in the Mail programme on the iMac.

I have connected and disconnected to iCloud several times.

Rebooted several times also plus rebuilt Mail.

Incoming and outgoing servers are via iCloud.

I also added a new Mailbox under iCloud, shows correctly but no others default in. It says "hide" not "show" by the side of the folders.

I am perplexed.

Any suggestions, please?

All help appreciated