This is to do with the WIP already having been allocated to a write off posting, if you follow the below to check if there are any batches not posted:
Open General Postings from the Main Menu.
Check that there isn't a WOW batch at the right hand side showing to be posted.
Open Slip Postings and check the To Post/Rejected/Unauthorised tabs at the top.
If there are WOW postings and they relate to the matter you are doing either Post or Delete to redo the WOW posting.