Strange Query Result

Greetings,

I have created an Access database using 2010 MS Access that has a Table with data that was imported from Excel.

I created a query to pull records where the [first_name] is blank or "is null". What I get is 10 of 13 records. For some strange reason, it doesn't pull up the other three blank records. Further, when using * under criteria, it pulls up all records, including the blank ones under [first_name].

Any idea or thoughts why this is happening?

Thanks in advance for your attention..

-Tommy