How are you modifying the report and what changes did you make? What does "can't" mean - it gives an error, gives a blank report, the action does nothing, the action is no longer there? Something else?

After verifing, the report (invoice) is not genrated for only one specific custormer. for other customers, it can be genrated and printed. May be that the report is requesting some informations that is not found for that customer.