Unable to change Request to re-assign to a new technician, on some Requests

Hello,
I'm currently experiencing a sporadic issue where on about one third of Requests that have been assigned to a technician A in my group. I am unable to assign the request to another technician B in the same group.

There are no error messages on screen, once you click save the message "Assigned Successfully" appears, but the technician has not changed and the history shows "Assigned from technician A to technician A". The problem is repeatable for the same Request.

All the Requests where I have observed this is happening seem to be in status "Awaiting Customer" and there has been editing and correspondence with the customer, however other Requests that are also Awaiting Customer and also have correspondence allow the technician to be re-assigned without a problem.

Any ideas what could be causing this?

Running Version : 9.4 Build 9417

Thanks
Jon