Answer:
Company X purchases a large volume of the goods that your company sells, therefore they are great clients for the sales department. On the other hand, they are very slow when it comes to paying their debts, therefore the accounts receivable department considers them a bad client.
This happens when each department is only able to view the information regarding their own activities and it is unable to access information about other departments' activities. A good way to solve this would be to use an ERP management software where the company's information could be shared between departments.