iCloud putting valid mail in junk folder

Really stuck with mail / iCloud moving valid mail to the junk folder. My situation is as follows :-

A number of mails I want, are being placed in the junk folder, but are not being marked as junk. I have no rules set on either mac, phone or iCloud that could be causing these movements. Actions I have carried out so far include :-

1) Verified no rules are causing the mails to be moved. Both using settings on iCloud web site and within the Mail for Mac app.

2) I have tried marking them as junk, so that I can use the unjunk button. But the next mail from the sender ends up back in the junk folder

3) I have tried adding the email address to the VIP list. This is ignored.

The whole thing is making the junk mail process useless as I am spending so much time checking the junk folder for incorrectly categorised mail.

Any ideas would be greatly appreciated.