Would anyone know why the single occurrence ‘Name’ field would grey out??

We are creating the quotation from business opportunity - 9 times out of 10 it works okay, but occasionally the quote greys out the name field on the single occurrence and we cannot amend. So if we want to put a contact/contractor name in the single occurrence we are unable to.

The work around is to open up a new sales quotation screen, re-enter the quote number and it frees up the field! Any ideas:

Example 1 - Single Occurrence unchecked - delivery = document address - all as expected :)

Example 2 - Check the Single Occurrence box - name and address is now greyed out!

Only way to clear this issue is to open a NEW window, look up the sales quotation

Example 3 - New window, now I can enter name/address after the single occurrence box is checked

Any ideas what is causing the screen to lock the field??? What can I do to stop this.

Driving me nuts as no errors at all and cant see any background processes holding it up. It doesn't free up no matter how many times we refresh