Maintenance WO Status Change Issue

Hi all

I have an issue with maintenance WO's, where despite two different WO's having largely the exact same setup such as Order, WO type and activity rules I can only move one of them for one specific status to another.  I am trying to assist with moving a maintenance WO from a status of MZ to MX which is allowed accoridng to the activity rules setup, as below:

What am I missing?  Why would I be able to move one of the maintenance WO's and not the other?  Any help will be appreciated.

Thanks

Nic