There are a few things to check in this case:
1) Does the client have the 'NDIA Claims' selected as there primary / additional recipient in there profile?
2) Do the clients have billable entries with the status 'Unpaid' in there profile? If there have a status of 'Draft' or 'NDIS Draft', this means the billable entries are already included in a NDIS generation which hasn't been submitted, or in a bulk invoice draft.
3) Have you synced any billable tasks for this generation? If not, see here for information on how to do that.