Occassionally, for an unknown reason, an operation may show as having 'Parallel Generate' status, but have a blank ' Edit Status'. The operation cannot be re-generated or edited.
A workaround is to select the parent program group, and 'MB3 - parallel generate'. This will re-generate all of operations including the problem operation.
(Note that it is not sufficient to multiple select all of the operations, the parent program group must be selected.)