You can go into station administrator and run database verification. You can look for errors in the client table and the slip table. Alternatively, you might start by running a slip detail report for the clients in question. My guess is you might have some slips that don't have a valid task.

If they were already billed, then database verification might fix, otherwise might take some behind the scenes work. Let us know if we can help. 

Could it be that one (or more) of your Custom fields is marked as "This is a required field"?

Thank you.  The database verification fixed it.  Not sure how but I'll take the win!