Unable to add AR Credit with serial numbered items

Forum,

I have the following scenario on a 2007A SP01 PL11 system:

I have an AR Invoice which has the status Open-Printed. This item has been returned by the customer so I go into 'Copy To' and select AR Credit Note.

Within the AR Credit Note I try to click on 'Add' and get prompted with the 'Choose SN and Batches' screen, which is what I'd expect considering these items are serially managed. I then click on the arrow to the left of the quanity value and get prompted by the 'Serial Number Selection' screen. The serial numbered item is sitting on the right hand side under 'Selected serial Numbers' section. I highlight it, then click on the < option to move under the section headed 'Available Serial Numbers'. I click on 'Update' and then ok to return to the AR Credit Note screen. I then click on 'Add' and have to go through the same process again and again. The only way out is to cancel the AR Credit Note.

Any ideas why this would be the case?

Regards,

Juan