Logic looks fine. The resultant SQL also should be fine. So, it maybe to do with the data values for this custom property. Use the Report > Show SQL option from the Report Writer designer view, copy the SQL into a new Advanced SQL report and modify this line in the WHERE clause
Cust <> 'NewCust'
to
Cust NOT LIKE '%NewCust%'
If this returns the data you need, take a closer look at the custom property values.