Hi Eloise,
The write-off GL account is determined from the Control Account Set, associated with the customer. It may be that if you edit your Control Account Set records, the write-off account is blank. There shouldn't be any blank accounts here, as that will result potentially in 'one-sided' entries.
GL shouldn't allow posting these batches, should this scenario occur. You still should be able to correct these batches in GL.
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Regards,
Softrak Tech Support