Typically this is due to a datatype mismatch or data length not matching in the virtual tables what is coming back from the database. For example if the column is set to an integer and there is an alpha character that is returned it can cause it to not populate correctly. Or the column is set to 50 and there are 100 characters coming back you may run into the same issue.
If you have the output view activated you may see an error message in the output view when running a query built against the virtual table. Are there any error messages in the output view when you run a query that is built against the virtual table? If so, can you copy those messages into this thread?
If you right click on the Virtual Table, select Properties and then click Structure, verify that the correct data types and lengths are set.
If the structure is correct and there are no errors in the output view, we will need to collect the support data to see if there is anything being logged.